• The World Health Organization is investigating reports of blood clots in some people who received the AstraZeneca Covid-19 vaccine.
• At least nine countries have now suspended their use of the vaccine over safety concerns.
• AstraZeneca said in a statement Friday that there's "no evidence" that the vaccine causes an increased risk of developing blood clots.

The World Health Organization said Friday it is reviewing recent reports of blood clots in some people who received the AstraZeneca Covid-19 vaccine that have prompted a few countries to halt their rollout of the shots.

At least nine countries, including Denmark, Norway, Iceland and Thailand, have suspended their use of the vaccine over safety concerns. As of Wednesday, around 5 million people in Europe had received the Oxford-AstraZeneca vaccine. Of this figure, 30 so-called thromboembolic events have been reported in recipients. Those are blood clots that form in blood vessels and block blood flow.

AstraZeneca said in a statement Friday that there's "no evidence" that the vaccine causes an increased risk of developing blood clots.

WHO Director-General Tedros Adhanom Ghebreyesus said Friday that the agency's Global Advisory Committee on Vaccine Safety "systematically reviews safety signals and is carefully assessing the current reports on the AstraZeneca vaccine."

"As soon as WHO has gained the full understanding of these events," he added, "the findings and any changes to our current recommendations will be communicated immediately to the public."

Dr. Mariangela Simao, WHO assistant director-general for access to medicines and health products, added that the global health agency will "probably have a statement on this next week as investigations are completed."

"WHO is very much aligned with the position that we should continue immunization until we have clarified the causal relationship," she said.

Dr. Soumya Swaminathan, WHO's chief scientist, said it remains unclear if the vaccine is actually causing the blood clots. An AstraZeneca spokesperson noted that "the observed number of these types of events are significantly lower in those vaccinated than what would be expected among the general population."

"The adverse events which are reported after vaccination have to be seen in the context of events which occur naturally in the population," Swaminathan said. "Just because it's reported following a vaccination doesn't mean that it's because of the vaccination. It could be completely unrelated."

Europe's drug regulator, the European Medicines Agency, has stressed that there is no indication the AstraZeneca shot is causing blood clots, adding it believes the vaccine's benefits "continue to outweigh its risks."

"Reports of blood clots received so far are not greater than the number that would have occurred naturally in the vaccinated population," said Dr. Phil Bryan, vaccines safety lead at Britain's Medicines and Healthcare Products Regulatory Agency.

"The safety of the public will always come first. We are keeping this issue under close review but available evidence does not confirm that the vaccine is the cause. People should still go and get their COVID-19 vaccine when asked to do so," he added.

— CNBC's Sam Meredith contributed to this report.