There has been an explosion outside the Kabul airport, the Pentagon press secretary said on Thursday, adding that it was unclear whether there were casualties amid the large evacuation effort in Afghanistan’s capital.

U.S. President Joe Biden has been briefed on the explosion, according to a White House official. Biden was in a meeting with security officials about the situation in Afghanistan, where the United States is in the final steps of ending its 20-year war, when the explosion was first reported, according to a person familiar with the matter.

The United States has been racing to airlift its citizens and some Afghan citizens from Kabul before its military is set to fully withdraw from Afghanistan on Aug. 31.

The explosion came hours after British Armed Forces Minister James Heappey said there was “very credible” intelligence that militants were planning an attack on people gathering at the airport, foreign media reported.

Western nations had warned of a possible attack on Kabul’s airport in the waning days of the massive evacuation efforts.

Several countries urged people to avoid the airport, where an official said there was a threat of a suicide bombing. But just days — or even hours for some nations — before the evacuation effort ends, few appeared to heed the call.

Over the last week, the airport has been the scene of some of the most searing images of the chaotic end of America’s longest war and the Taliban’s takeover, as flight after flight took off carrying those who fear a return to the militants’ brutal rule.

Already, some countries have ended their evacuations and begun to withdraw their soldiers and diplomats, signaling the beginning of the end of one of history’s largest airlifts. The Taliban have so far honored a pledge not to attack Western forces during the evacuation, but insist the foreign troops must be out by America’s self-imposed deadline of Aug. 31.

-Agencies