The head of the us food and drug administration has risked sparking a fresh conflict with donald trump over the timing of a coronavirus vaccine, after insisting his scientists would stick to a set of guidelines that the us president has threatened to overrule.

Stephen hahn, fda commissioner, said on tuesday his officials would assess any vaccine application against a set of criteria that experts say make it all but impossible that one will be available before novembers election.

Those guidelines which include a requirement that companies monitor half their trial participants for at least two months before making an application have been the source of political contention since the president said last week he might not approve them. mr trump called them a political move.

Mr trump has insisted a vaccine will be available within weeks, but scientists worry that rushing through an approval process could compromise both safety and public trust in a vaccine.

Speaking at the financial times us pharma and biotech summit, dr hahn said: the criteria that weve established for a vaccine, are the criteria we are going to use. weve been very clear about that with sponsors, very clear about that with the public. they are criteria that our career scientists are going to use to make a decision about the safety and effectiveness of the vaccine.

The fdas role in authorising a coronavirus vaccine has put it in the middle of one of the most highly charged debates in us politics. the presidents advisers regard getting a vaccine authorisation before an election as the holy grail, but joe biden, mr trumps presidential rival, has warned that rushing through an approval could compromise safety.

Dr hahn told the ft earlier this year he was willing to grant an emergency authorisation to a vaccine before the end of phase 3 clinical trials, prompting speculation that one could be released imminently.

But the fda agreed a set of guidelines with pharmaceuticals companies, the details of which emerged this month. the guidelines are not intended as hard-and-fast rules, which makes it difficult for the president to overrule them.

Dr hahn on tuesday stressed their importance in boosting public confidence in any vaccine. weve been very transparent about those criteria, we stick to those criteria youve seen that in the press and we continue to stand by those criteria, he said.

When a decision is made if a decision is made [people will be able to] look at that and assess those data in the context of the criteria that we set out.