The move comes a week after European regulators said they had found a possible link between AstraZeneca's COVID-19 vaccine and a similar rare blood clotting problem that led to a small number of deaths.

Johnson & Johnson's (J&J) single dose vaccine - most COVID-19 shots are delivered over two doses - and AstraZeneca's low-cost vaccine are seen as vital tools in the fight against a pandemic that has claimed more than three million lives around the world.

Most of the available J&J vaccine has been used in the United States due to production issues that have limited the company's supply.

As of April 12, more than 6.8 million doses of the J&J vaccine had been administered in the United States, compared with more than 180 million shots combined of the Moderna and Pfizer-BioNTech shots.

An advisory committee to the US Centers for Disease Control and Prevention (CDC) will meet on Wednesday to review the cases, and the US Food and Drug Administration (FDA) will review the analysis, the agencies said in a joint statement.

All six cases involved women between the ages of 18 and 48, and the symptoms occurred six to 13 days after vaccination.

In the cases, a type of blood clot called cerebral venous sinus thrombosis (CVST) was seen in combination with low levels of blood platelets (thrombocytopenia).

The CDC and FDA said the adverse events appeared to be extremely rare.

"I think this is a very low risk issue, even if causally linked to the vaccine: 6 cases with about 7 million doses (lower than the risk of clots with oral contraceptives) is not something to panic about," Dr Amesh Adalja, an infectious disease expert at the Johns Hopkins Center for Health Security in Baltimore, said in an email.

"People are asking me if they should cancel their J&J vaccine appointments and I have told them not to but I know many will and this will stall progress in controlling the pandemic."

J&J said it was working closely with regulators and noted no clear causal relationship had been established between the cases and the COVID-19 vaccine made by its Janssen unit.

One woman died and a second in Nebraska has been hospitalised in a critical condition, the New York Times reported, citing officials.

"The FDA recommendation to pause the administration of the Johnson and Johnson vaccine out of an abundance of caution makes sense in terms of the nature of the unusual and serious side effect not seen with the other vaccines," said Dr Robert Klugman at the UMass Memorial Medical Center in Massachusetts in an email.

"While the incidence is very low, the severity and potential for brain damage and other blood clot-related injuries is of great concern."

The US move comes less than a week after Europe's drugs regulator said it was reviewing rare blood clots in four people in the United States who had received the J&J shot.

The J&J and AstraZeneca vaccines both use an adenovirus vector - a harmless cold virus that instructs human cells to produce a protein found on the surface of the coronavirus, thereby spurring the immune system to prepare an arsenal against the COVID-19-causing virus.

Among leading global COVID-19 vaccine developers, China's CanSino Biological and Russia's Gamaleya Institute with its Sputnik V vaccine are also relying on this approach.

The Pfizer-BioNtech and Moderna vaccines use mRNA technology.

Australia has no current plans to add J&J's coronavirus vaccine to its immunisation drive, authorities said on Tuesday, as it moves away from procuring vaccines under review for potential links to blood clots.

Australian Associated Press