The U.S. Embassy in Kabul said on Sunday that the security situation at the airport in the Afghan capital was changing quickly, and that there were reports of gunfire as U.S. troops aided the evacuation of U.S. personnel.

The embassy said this in a security alert.

“There are reports of the airport taking fire; therefore we are instructing U.S. citizens to shelter in place,” the embassy said.

A source familiar with the situation could not confirm reports of firing there.

Secretary of State Antony Blinken had said earlier on Sunday that embassy staff were leaving the diplomatic compound and moving to the airport as Taliban insurgents entered Kabul.

Diplomats were being ferried by helicopter to the airport, where U.S. troops were providing security amid an exodus of Americans and their local allies and other foreigners.

Blinken insisted that the situation in the Afghan capital was “manifestly not Saigon.”

Taliban fighters had reached Kabul “from all sides”, a senior Interior Ministry official told Reuters and there were some reports of sporadic gunfire around the city.

Sources had told Reuters that most U.S. staff were expected to be evacuated from Kabul in the coming day or two.

“We’re working to make sure that our personnel are safe and secure.

“We’re relocating the men and women of our embassy to a location at the airport,” Blinken told ABC news.

Asked if the evacuation was evocative of the U.S. departure from Vietnam in 1975, he said: “Let’s take a step back. This is manifestly not Saigon.”

A NATO official said the group was maintaining its diplomatic presence in Kabul and helping to keep the airport running.

France, Germany and the Netherlands, all NATO members, said on Sunday they were moving their diplomats from their embassies.

A U.S. intelligence assessment earlier in the week had said Kabul could be encircled in 30 days and could fall to the Taliban within 90 days, but the insurgents captured most of Afghanistan’s major cities in less than a week and entered the capital on Sunday.

Some 4,200 people remained in the U.S. embassy until Thursday, when the Taliban’s rapid gains forced the Biden administration to begin flying in thousands of troops to help pull out many of the remaining diplomats.

The State Department insisted at the time that the embassy was not closing, but by Sunday sources said almost all U.S. personnel would be withdrawn from the country within a day or two.

U.S. officials said they were weighing whether more troops were needed to help with the evacuation, but added that at some point capacity would become an issue at the airport. Roughly 5,000 troops have already been authorized, but another 3,000 are on standby in Kuwait.

Washington invested billions of dollars over four U.S. administrations in Afghan government forces, giving them advantages over the Taliban, but they were unable to defend the country in the face of the militants’ advance, Blinken told CNN.

“That has happened more quickly than we anticipated,” Blinken said.

The United States’ original mission in Afghanistan, launched to oust al Qaeda chief Osama bin Laden after the Sept. 11, 2001, attacks, had been fulfilled, Blinken said, saying Washington had prevented further attacks by militants harbored by the Taliban.

But U.S. President Joe Biden has faced rising domestic criticism after sticking to his plan to withdraw, which was agreed under his Republican predecessor Donald Trump.

Republican lawmaker Michael McCaul said on Sunday that a Taliban takeover would revive the threat to the United States.

“We are going to go back to a pre-9/11 state. A breeding ground for terrorism,” he told CNN.

U.S. Senate Majority Leader Chuck Schumer, a Democrat like Biden, said Washington’s top focus should be on safely getting U.S. personnel and Afghans who supported the United States out of the country.