Federal health agencies in the US on Tuesday recommended a pause in the use of Johnson & Johnson's Covid vaccine in the country "out of an abundance of caution" after six recipients developed a rare and severe type of blood clot.

All six cases occurred among women between the age group of 18 and 48, and the symptoms occurred six to 13 days after vaccination.

The US Food and Drug Administration (FDA) and the Centres for Disease Control and Prevention (CDC) have urged the states across the country to pause the use of the single-dose vaccine while they investigate cases of blood clots.

The US CDC will convene a meeting of the Advisory Committee on Immunization Practices (ACIP) on Wednesday to further review these cases and assess their potential significance while the FDA will review that analysis as it also investigates these cases.

"Until that process is complete, we are recommending a pause in the use of this vaccine out of an abundance of caution," the CDC and the FDA said in a joint statement.

"This is important, in part, to ensure that the healthcare provider community is aware of the potential for these adverse events and can plan for proper recognition and management due to the unique treatment required for this type of blood clot," it said.

As of April 12, more than 6.8 million doses of Johnson & Johnson Covid-19 vaccine have been administered in the US.

In the six reported cases in the US, a type of blood clot called cerebral venous sinus thrombosis was seen in combination with low levels of blood platelets.

Treatment of this specific type of blood clot is different from the treatment that might typically be administered.

Usually, an anticoagulant drug called heparin is used to treat blood clots. In this setting, administration of heparin may be dangerous, and alternative treatments need to be given, the CDC and FDA joint statement said.

"Right now, these adverse events appear to be extremely rare," said the statement.