An explosion took place outside the Kabul airport on Thursday as the United States and other countries try to evacuate their citizens and Afghans at risk from the Taliban.

The blast happened at one of the entry gates of the Hamid Karzai International Airport and appears to be a suicide attack, according to three US officials.

Crowds of Afghans have been gathering at the gates trying to gain access to the airport, which has become the only way out of the country since the Taliban took over Kabul.

Tens of thousands of people have been evacuated by the US military and NATO allies from the airport in the past two weeks. These evacuations are set to start winding down in the next few days, ahead of the August 31 deadline for the final exit from a 20-year war in Afghanistan.

One US official said there are injuries among Afghans, but there is no information yet on any US casualties.

Pentagon spokesperson John Kirby confirmed there was an explosion outside of the airport. “We can confirm an explosion outside Kabul airport. Casualties are unclear at this time. We will provide additional details when we can,” he said in a tweet.

US officials said earlier on Thursday that concerns about security around the airport have increased based on “a very specific threat stream.”

The top US diplomat in Kabul, Amb. Ross Wilson, said Thursday that he could not get into the specifics of the security threat outside the gates of the Kabul airport cited in an embassy alert, but noted that “it was clearly regarded as credible, as imminent, as compelling.”

US President Joe Biden has been briefed on the explosion outside Kabul airport, a White House official said.

A spokesperson for the UK government said the government was “working urgently to establish what has happened and its impact on the ongoing evacuation effort.”

“Our primary concern remains the safety of our personnel, British citizens and the citizens of Afghanistan. We are in close contact with our US and other NATO allies at an operational level on the immediate response to this incident.”