• One week into Russia's invasion of Ukraine and military and defense analysts are united on one front: Russia's invasion has not gone entirely to plan.
• Analysts believe Russia had expected to make far more gains and, crucially, to face far less resistance from Ukrainian forces and volunteer fighters.
• Russian forces have attacked various towns and cities on the north, east and south of the country.
• A week into the invasion and Russian forces have claimed one major city — Kherson.

One week into Russia's invasion of Ukraine and military analysts are united on one front, at least: Russia's invasion has not gone entirely to plan, looking disorganized, uncoordinated and sluggish to observers.

Analysts believe Russia had expected to make far more gains and, crucially, to face far less resistance from Ukrainian forces and volunteer fighters as they attacked various towns and cities on the north, east and south of the country.

Russian forces have so far claimed one major city — Kherson — having heavily shelled and surrounded the port in the last several days.

Meanwhile, fighting continues around other major cities Kharkiv, Mariupol and Kyiv, although a huge column of Russian military vehicles which has been snaking its way to the capital this week has appeared to have stalled in recent days amid unconfirmed reports of logistical problems and food and fuel shortages.

Summing up Russia's challenges, senior military fellows at the Atlantic Council think tank stated in an online post on Wednesday that Russia had made key strategic errors in its first week of combat, particularly in its failure to establish air superiority and thus provide air support to its ground forces.

"During the first week of the war, Russian ground forces have become bogged down outside of the northern Ukrainian cities of Kharkiv and Kyiv due to their failure to establish air superiority (which has resulted in significant aircraft and helicopter losses), too few troops to execute three simultaneous thrusts (toward Kyiv and Kharkiv, and north from Crimea), poor coordination of fires and maneuver, significant logistical issues, and stronger than expected Ukrainian resistance," they said in an assessment published by the think tank.

The military experts acknowledged, however, that "Russia's naval superiority in the Black Sea has contributed to success in its southern area of operations, with Russian forces breaking out from the Crimean Peninsula and taking territory in southern Ukraine," the seizing of the port city of Kherson being its most major victory so far during the invasion.

They noted that "although Ukraine has fought well and disrupted plans for a quick and decisive Russian victory, the situation is still perilous. Russia is moving to encircle Kyiv and Kharkiv and appears to have switched to indiscriminate long-range fires — resulting in significant collateral damage in residential areas— and is making significant progress in the south."

Behind, or on, schedule?

Western intelligence officials have suggested that Russia's invasion is behind the Kremlin's schedule and there have been reports that Russian President Vladimir Putin has become increasingly frustrated by Russia's military struggles in Ukraine, current and former U.S. officials briefed on the matter told NBC News.

They also warned that Putin may see his only option as doubling down on the violence Russia unleashes against the country and a number of analysts have questioned Putin's rationality when it comes to Ukraine.

Without access to Putin's inner circle, Russia's schedule for its Ukraine invasion is largely guesswork and on Thursday, Putin insisted everything is "going to plan," stating that "all objectives that were set are being resolved or achieved successfully."

He also again reiterated Russia's aims, being the "demilitirization and denazification" of Ukraine — a statement widely disputed and rubbished and seen as Russia's attempts to vilify the Ukrainian leadership — and promised compensation for the families of dead and wounded servicemen.

Begging to differ with Putin's assessment (or propaganda) on Russian progress in the invasion, former CIA director Gen. David Petraeus, said this week that Putin's war in Ukraine is "going terribly" for Russia, telling CNN on Wednesday that "at the strategic level, he has essentially united most of the rest of the world ... And then on the battlefield, it's going terribly."

He said Russia was "stretched beyond its logistical and mechanical capabilities," its troops (some of whom are less experienced conscripts) likely to be extremely tired and inexperienced in the face of a determined opponent, as Ukraine is proving to be.

That point is agreed with by Col. Liam Collins, founding director of the Modern War Institute in New York, who said Thursday that Ukraine's army, and thousands of volunteers who have stayed in Ukraine to fight to save their homeland, would continue to mount a staunch resistance in Ukraine's capital, Kyiv.

"Not only do the Russians have to fight through the Ukrainian forces that are there, they have to fight through all these armed volunteers that are just going to be conducting attacks on them the whole time," he told the BBC's "The Briefing Room" show, adding that Ukrainians were preparing to mount an insurgency against Russia.

"It's going to be worse than what the Russians had in Afghanistan, that's what the Ukrainians are going to do," he added, alluding to the then Soviet Union's drawn-out, bloody, costly and unpopular invasion of Afghanistan in 1979 that lasted 10 years and led to the loss of around 15,000 Soviet soldiers.

"It's not going to be the minimal resistance that the Baltic states put up in the Cold War. It's going to be extremely costly if he's [Putin] going to be an occupier and so ultimately he's going to have to leave whether it's in one year, or five years or 10 years," he said.

While Ukraine's forces and volunteer fighters appear determined to confront Russian troops approaching Kyiv in a huge military convoy, over a million people are now deemed to have fled the country. Civilian casualties in Ukraine have prompted some Western officials, like British Prime Minister Boris Johnson, to accuse Putin of war crimes.

Accurate data on casualties and the injured, as well as the loss of military hardware, are hard to come by in war, with both sides having a vested interest in aggrandizing their own victories and accomplishments and belittling those of the opponent. Crucially, both sides have an interest in minimizing their losses too as they strive to maintain the morale of troops and their respective publics, alike.

As such, establishing an accurate death toll in the Russia-Ukraine crisis is difficult in the chaos of the conflict but Ukraine claimed on Wednesday that over 5,000 Russian personnel had died in the conflict while Russia's defense ministry said on Wednesday that 498 Russian soldiers had died and another 1,597 had been wounded.

Russia took several days to even acknowledge, and concede, that some of its personnel had been wounded and killed, with one military analyst telling CNBC Wednesday that he believed Russia "thought it would be completely easy" to invade Ukraine.

"[They thought] they would roll right in and the Ukrainians would give up," Jack Jacobs, a retired colonel in the United States army, told CNBC's "Squawk Box Asia" on Wednesday, saying Russia had underestimated the tenacity of ordinary Ukrainian people.

Russia has not been able to swiftly achieve its military objectives, he said, because "the Russians are not nearly as well trained as they think they are or as we thought they were, they're not nearly as well equipped," he said.

Overall trend 'still unfavorable'

Ukraine is not a member of NATO, the Western military alliance is not obliged to defend it, although a number of Western countries have sent weapons to Ukraine to help it defend itself.

Despite a resistance that has won hearts and minds around the world, the bigger picture does not look good for Ukraine, one analyst noted, and Ukraine needs more Western help if it is to stop Russia's slow but destructive and demoralizing advance.

"While the advancement of Russian forces appears to be slow, costly, and challenging, the overall trend is still unfavorable for Ukraine," Andrius Tursa, Central and Eastern Europe advisor at Teneo Intelligence, said in an emailed note Thursday.

"Unless the West significantly steps up its military support or if there are mass defections/disobedience in the Russian armed forces, the latter holds more chances to prevail in the longer term, given its considerable advantages in multiple domains."

"Control of the capital Kyiv and the survival of President Volodymyr Zelensky's administration are crucial for the morale and defensive positions of the Ukrainian side. And even if Russia's military advancement stalls, Putin could use increasingly powerful weapons systems or even nuclear threats to force Kyiv to capitulate," he noted.

Looking further ahead, analysts agree that even if Russia "wins" in Ukraine, that will be the easy part, and holding the country — whose population predominantly has a pro-Western attitude and will be even more anti-Russian after the invasion — will be much harder.

"The extremely strong resistance from the Ukrainian army and local population reaffirms expectations that long-term occupation of large parts of the Ukrainian territory would be extremely challenging," Tursa said, noting that any new administration installed in Kyiv (as many analysts believe is part of Russia's plan) "would lack legitimacy and struggle to remain in control."

Other analysts agree. Tim Dowse, senior associate fellow at the Royal United Services Institute, said on Twitter on Wednesday that "despite all the visible failings, realistically it is hard to see how Russia will not eventually prevail militarily in Ukraine. The imbalance of forces is simply too great.'

"How will Russia — indefinitely — occupy, control and administer a very large country of 40m [million] overwhelmingly hostile people, with a ruined economy, a need for major reconstruction of damaged infrastructure and probably a serious humanitarian crisis?" he said.

Even if Russia was able to find Ukrainians willing to constitute a new administration, Dowse questioned whether civil servants, the police and other public officials would be willing to take orders from such people. He concluded: "Won't military victory be the start, not the end, of Putin's problems?"