Germany, France and Italy have halted rollouts of the Oxford-AstraZeneca vaccine, after a series of incidents in Europe involving blood clots.
They join several smaller European nations who have halted vaccinations as a precaution while checks are made.
The World Health Organization (WHO) has said there is no evidence that the incidents are caused by the vaccine.
It said it was reviewing reports relating to the jab, but it was important vaccinations continued.
It was good practice to investigate potential adverse events, it added.
There have been a number of cases in Europe of blood clots developing after the vaccine was administered.
However, experts say the number of blood clots reported after the vaccine were no more than those typically reported within the general population.
About 17 million people in the EU and the UK have received a dose of the vaccine, with fewer than 40 cases of blood clots reported as of last week, AstraZeneca said.
What measures are being taken?
Germany's health ministry announced on Monday that it would stop administering the Oxford-AstraZeneca vaccine with immediate effect, on the recommendation of the Paul Ehrlich Institute (PEI), the country's authority on vaccines.
"The background to this decision follows new reports of cases of cerebral vein thrombosis connected with an AstraZeneca vaccination," Health Minister Jens Spahn said.
"In light of these newly reported cases, the Paul Ehrlich Institute today re-evaluated the situation and recommended a suspension of vaccinations and further analysis."
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He said the decision was "not political". "All of us are very aware of the consequences of this decision, and we did not take this decision lightly," he added.
Shortly afterwards, French President Emmanuel Macron said France was suspending the vaccine until new advice was given by the European Medicines Agency (EMA) on Tuesday afternoon.
"We have a simple guide, to be informed by science and the competent heath authorities and to do it as part of a European strategy," he said.
And Italy's medicines agency extended a ban introduced on individual batches of the vaccine throughout the country, also pending the EMA decision.
The suspensions came less than a day after the Netherlands did the same. Its suspension will last until at least 29 March.
The Irish Republic, Denmark, Norway, Bulgaria and Iceland have also temporarily halted inoculations with the vaccine, while the Democratic Republic of Congo and Indonesia have postponed the launch of their rollouts.
Several European countries, including Austria, have suspended the use of certain batches of the drug as a precautionary measure.
Thailand announced that it would start using the vaccine on Tuesday, following a brief delay to the rollout over safety concerns.
Canadian Prime Minister Justin Trudeau said health experts had assured him all vaccines being administered in the country, including AstraZeneca's, were safe.
What do the WHO and other experts say?
WHO spokesman Christian Lindmeier said the body was investigating the reports.
"As soon as WHO has gained a full understanding of these events, the findings and any unlikely changes to current recommendations will be immediately communicated to the public," he said.
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"As of today, there is no evidence that the incidents are caused by the vaccine and it is important that vaccination campaigns continue so that we can save lives and stem severe disease from the virus."
The European Medical Association - which is also currently carrying out a review into incidents of blood clots - said the vaccine could continue to be administered.
The UK medicines regulator also said evidence "does not suggest" the jab causes clots, as it urged people in the country to get the vaccine when asked to do so.
Professor Andrew Pollard, director of the Oxford vaccine group that developed the Oxford-AstraZeneca jab, told the BBC's Today programme there was "very reassuring evidence that there is no increase in a blood clot phenomenon here in the UK, where most of the doses in Europe [have] been given so far".
While vast numbers of people are being vaccinated at pace around the world, some of them will still get sick with other things unrelated to the vaccine.
These pauses for the AstraZeneca vaccine are not because it is unsafe to give. It's to allow time for experts to explore why a small number of people who were recently given the shot also developed blood clots.
When an illness occurs shortly after vaccination, it is right to question whether the shot might have contributed in any way.
There is no indication or evidence, however, that the vaccine was linked or responsible.
In the UK, more than 11 million people have already received at least one dose of the AstraZeneca vaccine and there has been no sign of excess deaths or blood clots occurring. Europe's drug regulator has also backed the vaccine, saying its benefits are clear. Covid can be deadly and vaccination saves lives.
What did AstraZeneca say?
It said there was no evidence of an increased risk of clotting due to the vaccine.
It said that across the EU and United Kingdom there had been 15 events of deep-vein thrombosis (DVT) - a blood clot in a vein - and 22 events of pulmonary embolism - a blood clot that has entered the lungs - reported among those vaccinated.
AstraZeneca said these figures were "much lower than would be expected to occur naturally in a general population of this size and is similar across other licensed Covid-19 vaccines".
"The nature of the pandemic has led to increased attention in individual cases and we are going beyond the standard practices for safety monitoring of licensed medicines in reporting vaccine events, to ensure public safety," Ann Taylor, the firm's chief medical officer, said.
