"The deep state, or whoever, over at the FDA is making it very difficult for drug companies to get people in order to test the vaccines and therapeutics," Trump tweeted on Saturday.

"Obviously, they are hoping to delay the answer until after November 3rd. Must focus on speed, and saving lives!"

The message was directed at US Food and Drug Administration commissioner Stephen Hahn.

Trump has said in the past he hopes a vaccine is developed around the time of the election.

Experts, however, have warned that any political interference in the vaccine testing and approval process could cast doubt and undermine its safety and legitimacy.

Any concerns about the vaccine could lead to a lower level of uptake, making it harder to battle the pandemic.

Hahn has repeatedly emphasised that the regulator will stick to its usual stringent scientific processes and has said that it has not faced any pressure to alter its criteria.

Larger clinical studies are under way for several vaccine candidates, involving up to 30,000 subjects.

If these are completed by the northern autumn, the regulator could start the approval process, which is normally costly and lengthy.

US government experts, including the government's infectious disease specialist Anthony Fauci, have said they are confident a vaccine could be available in early 2021.

Australian Associated Press