The US Food and Drug Administration has recommended a "pause" to the rollout of the Johnson & Johnson COVID vaccine so it can investigate reports of blood clots.
Officials from the FDA and the Centres for Disease Control and Prevention (CDC) say they are looking into six reported cases "of a rare & severe type of blood clot" in women aged 18 to 48 in the days after they received the jab.
A total of 6.8 million doses of the single-shot Johnson & Johnson vaccine have been administered in the US. The company said that "no clear causal link" has been found between their vaccine and the clots.
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"Right now, these adverse events appear to be extremely rare," the FDA said in a statement on Twitter.
It said until the cases had been assessed for their potential significance, "we are recommending this pause".
"This is important to ensure that the health care provider community is aware of the potential for these adverse events and can plan due to the unique treatment required with this type of blood clot," it added.
It comes following the recent concerns over the link between the Oxford-AstraZeneca vaccine and blood clots.
The UK has not approved the Johnson & Johnson vaccine for use, but 30 million doses have been ordered.
In the US, the distribution network run by the federal administration - which includes mass vaccination sites - will pause the use of the vaccine.
The vaccination efforts run by states and other providers are expected to follow soon.
People who have had the Johnson & Johnson shot and experience a severe headache, abdominal pain, leg pain, or shortness of breath within three weeks have been told to seek medical help.
"We are aware that thromboembolic events including those with thrombocytopenia have been reported with COVID-19 vaccines," said Johnson & Johnson in a statement.
"At present, no clear causal relationship has been established between these rare events and the Janssen COVID-19 vaccine."
A meeting of the CDC's Advisory Committee on Immunisation Practices will take place on Wednesday to review the cases.
Findings from this review will be analysed by the FDA as they investigate the situation.
Analysis: Suspension ordered out of abundance of caution
By Thomas Moore, science correspondent
Unusual blood clots with a second vaccine - and intriguingly it uses the same 'viral vector' technology as the AstraZeneca shot.
It's far too soon to say there is a causal link with the Johnson & Johnson jab, but the US regulators have recommended a pause in its roll-out as a precaution.
Almost 7 million have been given the vaccine - and there have been six cases of a "rare and severe" form of blood clot.
The European regulators are also investigating cases.
Blood clots happen naturally, so the Food and Drug Administration will have to look carefully at the cases.
There is hypervigilance at the moment because of the concern over the use of the AstraZeneca vaccine in younger people.
Both vaccines use another virus to carry genetic material for the COVID spike protein into the body.
The Johnson & Johnson - also known as Janssen - vaccine isn't yet licensed in the UK, but 30 million doses are on order.
