"I can't predict when a vaccine will be available," US Food and Drug Administration (FDA) Commissioner, Dr Stephen Hahn, said on Sunday.
Dr Hahn said vaccine development would be "based upon the data and science".
A vaccine would train people's immune systems to fight the virus, so they do not become sick.
Dr Hahn, a member of the White House coronavirus task force, was asked about the timeframe after President Trump suggested that a "vaccine solution" to the pandemic would be ready "long before the end of the year".
"I want to send our thanks to the scientists and researchers around the country, and even around the world, who are at the forefront of our historic effort to rapidly develop and deliver life-saving treatments and ultimately a vaccine," Mr Trump said during his Independence Day address at the White House.
"We are unleashing our nation's scientific brilliance and we'll likely have a therapeutic and/or vaccine solution long before the end of the year."
The president has been criticised for his comments on vaccines and treatments during the coronavirus epidemic, which has claimed the lives of almost 130,000 people in the US.
In recent days, infections have been rising at a record rate in western and southern states, bringing the total to more than 2.8 million nationwide.
The head of the World Health Organization (WHO), Dr Tedros Adhanom Ghebreyesus, warned in June that scientists may never be able to create an effective vaccine against the coronavirus.
"The estimate is we may have a vaccine within one year," the WHO chief said. "If accelerated, it could be even less than that, but by a couple of months. That's what scientists are saying."
Other experts have suggested a Covid-19 vaccine will not be available until at least mid-2021.
What did Dr Hahn say?
In an interview with ABC News on Sunday, FDA chief Dr Hahn said "we are seeing unprecedented speed for the development of a vaccine", but did not elaborate on a timeline for its availability.
"Our solemn promise to the American people is that we will make a decision based upon the data and science on a vaccine, with respect to the safety and effectiveness of that vaccine," he said.
SOURCE: BBC
