Top US health authorities recommended a "pause" in the use of the Johnson & Johnson Covid-19 vaccine "out of an abundance of caution" as they investigate any links between it and blood clots, a regulator said Tuesday.

The US Food and Drug Administration and the Centers for Disease Control are assessing the "potential significance" of six reported cases of a rare blood clot in patients who have received the shot, the FDA tweeted.

"Until that process is complete, we are recommending this pause," it said.

The regulator said that as of Monday more than 6.8 million doses of the J&J vaccine had been administered in the US.

"CDC & FDA are reviewing data involving 6 reported U.S. cases of a rare & severe type of blood clot in individuals after receiving the vaccine. Right now, these adverse events appear to be extremely rare," the FDA stated on Twitter.

It said it was calling for the pause to allow healthcare providers to plan for "the unique treatment required with this type of blood clot."

The CDC will convene an advisory committee Wednesday "to further review these cases and assess their potential significance. FDA will review that analysis as it also investigates these cases," the tweets continued.

It said a news conference was scheduled for later Tuesday morning.

The statement came days after the European Union's drug regulator said it is also reviewing possible blood clot cases in people given the J&J shot.