When Washington decided to sever ties with the World Health Organization, top US health officials said they had not been consulted, and when President Donald Trump Saturday called for a slowdown in testing, his coronavirus taskforce said it had never been told to do so.

In response to questioning at a congressional hearing Tuesday if he was asked about the president's decision last month to withdraw the US from the WHO, Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said, "I was not specifically consulted about the withdrawal, or the attempt to withdraw."

Asked whether he has any concerns about the president's plans, Fauci replied, "Yes, I do.

"Despite any policy issues that come from higher up in the White House, we at the operational level continue to interact with the WHO in a very meaningful way … literally on a day-by-day basis," Fauci said at the House Energy and Commerce Committee's "Oversight of the Trump Administration's Response to the COVID-19 Pandemic" hearing.

Fauci was joined by Centers for Disease Control and Prevention (CDC) Director Dr Robert Redfield, Food and Drug Administration chief Dr Stephen Hahn and the head of the US Public Health Service, Admiral Brett Giroir, at the hearing, which lasted more than five hours.

Redfield also said he was "not directly" consulted with Trump's decision to terminate relations with the WHO. He said the CDC has a long history of working with the WHO and will continue to collaborate.

All four health officials testified that they had never been instructed to slow down testing for the coronavirus.

At a Saturday rally in Tulsa, Oklahoma, Trump said the US had tested 25 million people, far more than any other country. He said the "bad part" of it is that widespread testing leads to logging more caseloads, "So I said to my people, 'Slow the testing down, please.'"

White House officials have earlier said that Trump's remarks were in jest, but Trump told reporters on Tuesday, "I don't kid," when he was asked if he was joking.

"I know for sure none of us have ever been told to slow down on testing. That is just a fact," Fauci said. "In fact, we will be doing more testing."

The committee's chairman, Frank Pallone Jr, said Trump was "extremely reckless" in suggesting that the administration should slow down testing to reduce the number of positive coronavirus cases.

The US has logged more than 2.3 million coronavirus cases, with more than 120,000 deaths, by far the nation hardest hit by the novel coronavirus.

"Unfortunately, it continues the president's pattern of ignoring the advice of his own public health experts," Pallone said of Trump's rally comments, in his opening remarks at the panel hearing.

He said testing has been a problem since the beginning, and while it has improved, "we are still falling far short of the 900,000 daily tests public health experts believe we need".

"Had it not been for a sluggish initial response from the Trump administration and a president, in my opinion, putting political considerations over public health, we could have done much more to mitigate the destructive impact of COVID-19," Pallone said.

As the hearing was going on, Trump tweeted, "Cases up only because of our big number testing. Mortality rate way down!!!"

But Redfield testified that "several communities are seeing increased cases driven by multiple factors, including increased testing, outbreaks and evidence of community transmission", while Fauci cautioned, "Deaths always lag considerably behind cases. It is conceivable you may see the deaths going up."

Fauci said the New York metropolitan area, once hit "extraordinarily hard" by the pandemic, has done notably well in following federal guidelines and bringing cases down. "However, in other areas of the country, we're now seeing a disturbing surge of infections."

The US is "still in the middle of the first wave", he said, warning that "the next couple of weeks are going to be critical in our ability to address those surges".

Redfield added, "As we get to the fall, we're going to have influenza and COVID at the same time."