WASHINGTON (AP) —  President Joe Biden is pledging to Americans still trapped in Afghanistan: “We will get you home.”

Biden’s comments at a White House news conference Friday come as the U.S. government struggles to ramp up a massive airlift clearing Americans and other foreigners and vulnerable Afghans through the Kabul airport, rescuing them from a Taliban takeover of the country.

Biden is facing criticism for a chaotic and often violent scene outside the airport as crowds struggle to reach safety inside. He called the past week “heartbreaking,” but insisted his administration was working hard to smooth and speed the evacuations.

“I don’t think anyone of us can see these pictures and not feel that pain on a human level,” Biden said, but “now I’m focused on getting this job done.”

Evacuation flights at the Kabul airport had stopped for several hours on Friday because of a backup at a transit point for the refugees, a U.S. airbase in Qatar, U.S. officials said. However, a resumption was ordered in the afternoon, Washington time.

As many as three flights out of Kabul were expected in the next few hours, going to Bahrain and carrying perhaps 1,500 evacuees in all, said an official, speaking on condition of anonymity to discuss military.

In Washington, some veterans in Congress were calling on the Biden administration to extend a security perimeter beyond the Kabul airport so more Afghans can make it to the airport for evacuation. They also want Biden to make clear an Aug. 31 deadline for withdrawing U.S. troops is not a firm one.

The deadline “is contributing to the chaos and the panic at the airport because you have Afghans who think that they have 10 days to get out of this country or that door is closing forever,” said Rep. Peter Meijer, R-Mich., who served in Iraq and also worked in Afghanistan to help aid workers provide humanitarian relief.

Tens of thousands of people remain to be evacuated ahead of the United States’ Aug. 31 deadline to withdraw its troops from the country, although the pace had picked up overnight. A defense official said about 5,700 people, including about 250 Americans, were flown out of Kabul aboard 16 C-17 transport planes. On each of the previous two days, about 2,000 people were airlifted.

With desperate crowds thronging Kabul’s airport, and Taliban fighters ringing its perimeter, the U.S. government renewed its advisory to Americans and others that it could not guarantee safe passage for any of those desperately seeking seats on the planes inside.

The advisory captured some of the pandemonia, and what many Afghans and foreigners see as their life-and-death struggle to get inside. It said: “We are processing people at multiple gates. Due to large crowds and security concerns, gates may open or close without notice. Please use your best judgment and attempt to enter the airport at any gate that is open.”

While Biden has previously blamed Afghans for the U.S. failure to get out more allies ahead of this month’s sudden Taliban takeover, U.S. officials told The Associated Press that American diplomats had formally urged weeks ago that the Biden administration ramp up evacuation efforts.

In July, more than 20 diplomats at the U.S. Embassy in Kabul registered their concerns that the evacuation of Afghans who had worked for America was not proceeding quickly enough.

In a cable sent through the State Department’s dissent channel, a time-honored method for foreign service officers to register opposition to administration policies, the diplomats said the situation on the ground was dire, that the Taliban would likely seize control of the capital within months of the Aug. 31 pullout, and urged the Biden administration to immediately begin a concerted evacuation effort. That’s according to officials familiar with the document who spoke on condition of anonymity to discuss the internal debate.

Biden has said that the chaos that unfolded as part of the withdrawal was inevitable as the nearly 20-year war came to an end. He said he was following the advice of Afghanistan’s U.S.-backed president, Ashraf Ghani, in not earlier expanding U.S. efforts to fly out translators and other Afghans in danger for the past work with Americans. Ghani fled the country last weekend as the Taliban seized the capital.

Biden also said that many at-risk Afghan allies had not wanted to leave the country. But refugee groups point to yearslong backlogs of applications from thousands of those Afghans for visas that would let them take refuge in the United States.

The administration has also portrayed its contingency planning as successful after the Afghan government fell much faster than publicly anticipated by administration officials. Yet the White House received clear warnings that the situation was deteriorating rapidly before the current evacuation push.

The Kabul airport has been the focus of intense international efforts to get out foreigners, Afghan allies and other Afghans most at risk of reprisal from the Taliban insurgents.

With the Taliban controlling the Afghan capital, including the airport’s outer perimeter, White House national security adviser Jake Sullivan said that U.S. citizens are able to reach the airport, but were often met by large crowds at the airport gates.

On Thursday, Taliban militants fired into the air to try to control the crowds gathered at the airport’s blast walls. Men, women and children fled. U.S. Navy fighter jets flew overhead, a standard military precaution but also a reminder to the Taliban that the U.S. has firepower to respond to a combat crisis.

Sullivan acknowledged that there is the possibility of a hostage situation or terrorist attack, even though the government has been in contact to assure safe passage for U.S. citizens. The administration has committed to ensuring that all Americans can leave, even if that means staying past the August deadline.

“This is a risky operation,” Sullivan told NBC News Thursday. “We can’t count on anything.”

There is no firm figure of the number of people — Americans, Afghans or others — who are in need of evacuation as the process is almost entirely self-selecting.

The State Department says that when it ordered its nonessential embassy staff to leave Kabul in April after Biden’s withdrawal announcement, fewer than 4,000 Americans had registered for security updates. The actual number, including dual U.S.-Afghan citizens along with family members, is likely much higher, with estimates ranging from 11,000 to 15,000. Refugee advocates estimate about 100,000 Afghan allies and family members also are appealing for seats on the U.S. airlift.

Compounding the uncertainty, the U.S. government has no way to track how many registered Americans may have left Afghanistan already. Some may have returned to the United States but others may have gone to third countries.

Although Afghanistan had been a hotspot for the coronavirus pandemic, the State Department said Thursday that evacuees are not required to get negative COVID-19 results.

Additional American troops continued to arrive at the airport to safeguard and run the U.S. part of the evacuation. As of Thursday, there were about 5,200, including Marines who specialize in evacuation coordination and an Air Force unit that specializes in emergency airport operations. Biden has authorized a total deployment of about 6,000.