A suspected suicide bomb exploded outside Kabul airport on Thursday, killing at least 13 people including children, a Taliban official said, after the United States and allies urged Afghans to leave the area because of a threat by Islamic State.

The official said many Taliban guards were wounded.

A U.S. official said U.S. service members were among the wounded, adding he was citing an initial report and cautioning that it could change. He said there were casualties but did not know how many or of what nationality.

Thousands of people have been gathering outside the airport in recent days. Western troops are racing to evacuate foreigners and Afghans who helped Western countries during the 20-year war against the Taliban, and to get out themselves by an Aug. 31 deadline.

There were few details yet of the attack, but Western countries have been warning of a potential attack by Islamic State militants.

The Taliban, whose fighters are guarding the perimeter outside the airport, are enemies of the Afghan affiliate of Islamic State, known as Islamic State Khorasan (ISIS-K), after an old name for the region.

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 when the explosion was first reported, according to a person familiar with the matter.

The concerns about an attack came against a chaotic backdrop in Kabul, where the massive airlift of foreign nationals and their families as well as some Afghans has been under way since the day before the Taliban captured the city on Aug. 15, capping a lightning advance across the country as U.S. and allied troops withdrew.

“We wish we could have stayed longer and rescued everyone,” the acting chief of Canada’s defence staff, General Wayne Eyre, told reporters.

British Armed Forces Minister James Heappey said intelligence about a possible suicide bomb attack by IS militants had become “much firmer”.

“The threat is credible, it is imminent, it is lethal. We wouldn’t be saying this if we weren’t genuinely concerned about offering Islamic State a target that is just unimaginable,” Heappey told BBC radio.

Australia also issued a warning for people to stay away from the airport while Belgium ended its evacuation operations because of the danger of an attack. The Netherlands said it expected to carry out its last evacuation flight on Thursday.

Daesh, as it is widely known in Afghanistan, established a reputation for extreme brutality as it fought the Taliban both for ideological reasons and for control of local smuggling and narcotics routes, according to Western intelligence services.

The U.S. military has said it would prioritize evacuating its troops, numbering about 5,200, in the two days before the deadline to leave.

Since the day before the Taliban swept into Kabul, the United States and its allies have mounted one of the biggest air evacuations in history, bringing out about 95,700 people, including 13,400 on Wednesday, the White House said on Thursday.

U.S. Secretary of State Antony Blinken said at least 4,500 American citizens and their families had been evacuated from Afghanistan since mid-August.

The Taliban’s 1996-2001 rule was marked by public executions and the curtailment of basic freedoms. Women were barred from school or work. The group was overthrown two decades ago by U.S.-led forces for hosting the al Qaeda militants who masterminded the Sept. 11, 2001 attacks on the United States.

The Taliban have said they will respect human rights and will not allow terrorists to operate from the country.

REUTERS