Europe's medicines watchdog on Tuesday said the benefits of the AstraZeneca COVID-19 vaccine continue to outweigh risks after a growing number of countries halted its use due to concerns over reported cases of blood clots.

The European Medicines Agency (EMA) Executive Director Emer Cooke said there was no indication that the blood clot incidents, which he called "very rare" had been caused by the vaccine, but that experts were assessing that possibility.

Trust in the safety of the vaccines was paramount and the agency was carrying out a case-by-case evaluation, she said.

"The benefits continue to outweigh the risks, but this is a serious concern and it does need serious and detailed scientific evaluation. This is what we are involved in at the moment," Cooke told a news conference.

She noted that thousands of people across the EU develop thromboses every year for a variety of reasons and that there were no reports of increased blood clots in the clinical studies of the AstraZeneca vaccine. Still, experts would undertake a "very rigorous analysis," she said.

The results of its findings would be discussed during an EMA review on Thursday after which the results would be made public, Cooke said.

(With input from agencies)