The World Health Organization declared COVID-19 a pandemic Wednesday, as top U.S. public health officials warned lawmakers the virus’ spread will worsen in the coming weeks and called for aggressive mitigation.

Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told members of the House Oversight and Reform Committee that he expects the number of coronavirus cases to increase and that “serious mitigation” is necessary.

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“We will see more cases and things will get worse than they are right now,” Fauci told the panel. “How much worse [they] will get depend on our ability to do two things: to contain the influx of people who are infected coming from the outside and the ability to contain and mitigate within our own country.”

Fauci’s comments echoed those of WHO officials, who said in a separate news briefing that nations around the globe should work harder to both contain, or isolate, the virus and mitigate the disease’s impact. “We should be more aggressive,” said WHO Director-General Tedros Adhanom Ghebreyesus.

Congress is preparing a second legislative package to respond to the pandemic. House Democrats are set to release their proposal later Wednesday. House Speaker Nancy Pelosi, D-Calif., is negotiating a package with Treasury Secretary Steven Mnuchin.

Oversight and Reform members pressed the five federal health officials testifying about the administration’s response, lobbing questions about testing for the virus and whether large crowds of people should be allowed to form. The hearing recessed just before noon Wednesday, initially with plans to resume at 2 p.m.

But Chairwoman Carolyn B. Maloney, D-N.Y., later said Fauci and Centers for Disease Control and Prevention Director Robert Redfield had been “unavoidably detained” at the White House, and that the hearing would resume at 11 a.m. Thursday.

“We don’t know what is going on, but they cannot come back,” she said.

Some Democrats criticized the administration for the slow rollout of tests for the virus after a faulty control in the CDC’s test did not originally perform correctly and questioned why the U.S. did not use the same tests as in some foreign countries. Redfield told lawmakers that foreign tests like those used in South Korea could not be used in the United States until they were approved by the Food and Drug Administration.

Redfield also said the CDC isn’t currently planning to set up “drive-through” testing facilities to allow people to be tested for the virus.

“We’re trying to maintain the relationship between independent individuals and their health care providers,” Redfield said.

Fauci added a physician would likely direct patients who suspect they may have the virus to stay at home and instruct them on how to access a test so that they don’t infect additional people by going to a doctor’s office waiting room.

Some Democrats were especially critical of President Donald Trump’s actions. Massachusetts Democratic Rep. Stephen F. Lynch told witnesses they need to counter false statements from Trump about the virus, noting the president said anyone could access a test when that was not the case.

“When the president is making statements like this, we need pushback from the public health officials,” Lynch said. “Standing behind him and nodding silently or an eyeroll every once in a while is not going to get it.”

Fauci pushed back on that, noting he has worked for six presidents and said he has always stuck by scientific facts.

“I have never, ever held back telling exactly what is going on from a public health standpoint,” he said.

During the hearing, Rep. Mark E. Green, R-Tenn., ticked through the steps the Trump administration had taken to minimize the spread of the virus and criticized Democrats for politicizing it.

“The 24/7 criticism the president is undergoing is unwarranted at a minimum,” he said, adding it “absolutely maligns the hard work done over years” by doctors and scientists at federal agencies.