Republican leader Kevin McCarthy, who is vying to become speaker of the U.S. House of Representatives, and other Republicans have been pushing the Biden administration to lift the mandate, arguing in part that it hurts recruiting.

President Joe Biden, Defense Secretary Lloyd Austin and military leaders have strongly backed the vaccine mandate as necessary to safeguard the health and readiness of the U.S. armed forces.

"We continue to believe that repealing the vaccine mandate is a mistake," White House National Security Council spokesman John Kirby told reporters.

"Republicans in Congress have obviously decided that they'd rather fight against the health and well being of those troops, rather than protecting them," Kirby added.

A compromise version of the National Defense Authorization Act, which sets defense policy for the Pentagon, would scrap the vaccine mandate.

The bill is expected to pass the Senate and House this month, and be sent to the White House for Biden to sign into law.

Austin said on Monday that the military has no data to back up claims by top Republicans in Congress that the vaccine mandate is hurting recruiting.

Still, the U.S. military has faced unanticipated resistance among a minority of troops to getting vaccinated - something U.S. defense officials say is the result of misinformation about the safety of the vaccine.

According to Defense Department data, 3,717 Marines, 1,816 soldiers and 2,064 sailors have been discharged for refusing to get vaccinated.

Kirby said that more than 99 percent of active duty troops have received at least one dose of a COVID-19 vaccine.

The defense bill does not require that troops who refused the vaccine be reinstated but it does ask for a report on the issue.

Representative Adam Smith, the House Armed Services Committee's Democrat chairman, said the question involved discipline of a military which is expected to follow lawful orders.

"I think they have to take into account that these people refused direct orders," Smith said.