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

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

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 military commanders had told him an attack was highly likely in the next 24 to 36 hours. He said the situation on the ground remains "extremely dangerous".

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.