Out of nearly seven million Americans who have so far received the single dose vaccine, six women between age 18 and 48 developed a rare type of clot in the brain along with low platelets, officials said.

One later died while another is in critical condition.

Food and Drug Administration scientist Peter Marks said the disorder might be triggered by a rare immune response to the vaccine similar to that seen among a few hundred recipients of the AstraZeneca jab in Europe.

Both vaccines are based on adenovirus vector technology.

"We have made the decision to proactively delay the rollout of our vaccine in Europe," J&J said, in a fresh blow for the hard-hit continent which passed the threshold of one million coronavirus deaths.

The European Union has signed a deal for 200 million shots of the vaccine with an option for 200 million more.

US authorities are now carrying out an investigation which could end up leading to tough regulatory choices, such as restricting the J&J shot to older people.

People who have received a J&J shot within the past three weeks were asked to report to their doctors if they experienced severe headaches, abdominal pain, leg pain or shortness of breath.

But the White House said it was confident there would be no "significant impact" on the vaccination plan in the world's hardest-hit country, where almost half of all adults have now received at least one dose.

Pfizer said it would deliver its 300 million doses by the middle of July -- two weeks early -- while Moderna said it was on course to deliver its total of 300 million by the end of July.