The EU's drug regulator has said it has launched a review of possible links between the Johnson & Johnson coronavirus vaccine and blood clots after reports of four cases, one of them fatal.
It comes as US health authorities said they had found no "causal" link between the J&J vaccine and blood clots.
"At this time, we have not found a causal relationship with vaccination and we are continuing our investigation and assessment of these cases," said a statement from the US Food and Drug Administration.
The agency said it was aware of "a few individuals" in the United States who had suffered clots and low levels of platelets in the blood after receiving J&J's vaccine.
"Both conditions can have many different causes," the FDA said.
The European Medicines Agency (EMA) said its safety committee "has started a review of a safety signal to assess reports of thromboembolic events" with people who had received the shot.
"Four serious cases of unusual blood clots with low blood platelets have been reported post-vaccination with Covid-19 vaccine Janssen," the EMA said, referring to US pharma giant J&J's European subsidiary.
The watchdog has approved the Johnson & Johnson vaccine for use but its rollout across the 27-nation EU is not due to start until later this month.
Meanwhile, the EMA also said that it is investigating possible links between the AstraZeneca vaccine and cases of a rare blood vessel disorder.
The agency said it was reviewing five cases of capillary leak syndrome, which is "characterised by leakage of fluid from blood vessels causing tissue swelling and a drop in blood pressure".
The inquiry comes just days after the EMA said it was listing blood clots as a very rare side effect of the AstraZeneca vaccine.
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Both the Johnson & Johnson and AstraZeneca jabs use similar adenovirus vector technology.
The EMA said that after investigating both of the new reports it would "decide whether regulatory action may be necessary", which usually involves adding a notification about side effects.
The EMA said earlier this week that it was aware of cases linked to the J&J vaccine, but gave a lower number of three and did not mention the death.
The overall numbers were "extremely small" compared to the 4.5 million Johnson & Johnson shots administered worldwide, EMA safety chief Peter Arlett told a press conference on Wednesday.
"This is however under close scrutiny... I think it is fair to say there is intensive monitoring of this issue across the vaccines," Mr Arlett said.