President Joe Biden warned on Saturday that another militant attack was highly likely as U.S. troops began leaving Kabul airport after a two-week scramble by Washington and its allies to evacuate citizens and at-risk Afghans, Reuters reported.

As Washington neared the end of its military involvement in the country with the Taliban it ousted 20 years ago back in power, the United States said it killed two Islamic State militants planning attacks in Afghanistan after a deadly suicide bombing outside the airport on Thursday.

According to Reuters American officials also warned of a high risk of additional attacks by the group – enemies of both the West and Afghanistan’s new Taliban rulers – as U.S. forces wind up their mission before a Tuesday deadline set by Biden.

The U.S. president said the situation on the ground remained “extremely dangerous” and that military commanders had told him an attack was highly likely in the next 24 to 36 hours, Reuters reported.

After Thursday’s blast that killed scores of Afghans and 13 American troops, the most lethal incident for U.S. service members in Afghanistan in a decade, Biden vowed to hunt down the perpetrators. He said Friday’s strike was not the last.

“We will continue to hunt down any person involved in that heinous attack and make them pay,” he said.

The Taliban condemned the overnight U.S. drone strike, which took place in Nangarhar Province, an eastern area that borders Pakistan.

“The Americans should have informed us before conducting the air strike. It was a clear attack on Afghan territory,” a Taliban spokesman told Reuters, adding that two women and a child were wounded in the attack.

The Taliban have said they have arrested some suspects involved in the airport blast, reported said.

Spokesman Zabihullah Mujahid also said the Taliban would take over the airport “very soon”, after U.S. forces withdraw, and announce a full cabinet in the coming days.

The Western-backed government and Afghan army melted away as the Taliban entered the capital on Aug. 15, leaving an administrative vacuum that has bolstered fears of a financial collapse and widespread hunger.

Mujahid told Reuters the group had appointed governors and police chiefs in all but one of Afghanistan’s 34 provinces and would act to solve the country’s economic problems.

A U.S. official told Reuters on Saturday there were fewer than 4,000 troops left at the airport, down from 5,800 at the peak of the evacuation mission. Pentagon spokesperson John Kirby later confirmed to reporters that the withdrawal had begun but declined to say how many service members remained.

Thursday’s suicide blast, claimed by ISIS-K, locally known as Daesh, caused a bloodbath outside the gates of the airport – where thousands of Afghans have gathered to try to get a flight out since the Taliban returned to power.