Two explosions outside Kabul’s international airport caused at least 13 deaths, including some American troops, and wounded dozens of others less than a week before U.S. forces are due to depart.

“We can confirm that a number of U.S. service members were killed in today’s complex attack at Kabul airport,” Pentagon spokesman John Kirby said in a tweet. “A number of others are being treated for wounds. We also know that a number of Afghans fell victim to this heinous attack.”

The Wall Street Journal, citing an unnamed U.S. official, reported that the U.S. ambassador in Kabul told staff there that four U.S. Marines were among the dead.

The deaths of U.S. troops, after more than 100,000 people had been evacuated from Afghanistan in the past two weeks, will significantly raise the pressure on President Joe Biden to decide if he sticks with his Aug. 31 deadline to get all American forces out, and to explain why the U.S. appeared to be caught off guard by the Afghan government’s sudden collapse.

In addition to the U.S. troops, at least 13 Afghans were killed and 52 others wounded, Taliban spokesman Zabihullah Mujahed said by phone.

Kirby called the blast outside the Abbey Gate a “complex attack,” without providing details. It came after U.S. and NATO officials warned their citizens to avoid heading toward the airport. A U.S. official said flights departing Kabul have been temporarily halted.

The Taliban blamed the U.S., which is directing a military-led evacuation from Kabul, for drawing large crowds to the airport perimeter. Mujahed said an investigation into who was behind the attack is continuing, though many analysts said an offshoot of Islamic State — an enemy of both the Taliban and the U.S. — could be to blame.

“We strongly condemn this lethal attack which happened at an area of Kabul airport which is under the control of Americans,” Mujahed said. “They are responsible for the security of the area.”

The blast occurred around the time President Joe Biden was scheduled to meet with his national security team about the situation in Afghanistan. He has since been briefed in the White House Situation Room, according to an official. A late-morning meeting with visiting Israeli Prime Minister Naftali Bennett is now postponed, the White House said, and a planned virtual meeting with governors to discuss taking in Afghanistan refugees has been called off.

U.K. Prime Minister Boris Johnson is being kept updated on developments at Kabul’s airport and will host a meeting of the government’s emergency committee, later this afternoon, his office said in a statement. German Chancellor Angela Merkel canceled a planned trip to Israel over the situation in Afghanistan, her spokesman said.

“The explosion happened within a large crowd at the Abbey gate where people are being screened and processed by the Americans,” Mustafa Shah, an Afghan who was near the blast and took a wounded friend to the hospital, said in a phone call. Shah said he saw body parts on the ground and “10-15 people” who appeared to be dead.

Afghans and others trying to flee Kabul have packed around the airport trying to get onto one of the many military flights leaving the country.

After the explosion, European military officials sent a message to citizens in the country saying, “Get away from the airport. Very, very, very dangerous situation. Go now!” according to Dina Haynes, a lawyer who got a client into the Kabul airport compound a few minutes before the explosion.

While it wasn’t immediately clear who caused the explosion, earlier in the day, American and NATO allies had warned their citizens against traveling to the airport because of the credible and imminent risk of attacks. Biden specifically cited Islamic State — Khorasan, a terrorist group, as a potential threat, this week.

“They’re real and significant challenges that we also have to take into consideration the longer we stay, starting with the acute and growing risk of an attack by a terrorist group known as ISIS-K, an ISIS affiliate in Afghanistan, which is a sworn enemy of the Taliban as well,” Biden said Tuesday. “Every day we’re on the ground is another day we know that ISIS-K is seeking to target the airport and attack both U.S. and allied forces and innocent civilians.”

Biden this week reiterated his plan to withdraw all U.S. troops from Afghanistan by Aug. 31, though he called on the Pentagon and State Department to come up with contingency plans in case they are needed.

This report contains material from Bloomberg News and Reuters.