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.

Local media said three U.S. servicemen and some Taliban guards were among the injured. 

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 withdraw by an August 31 deadline.

(With input from Reuters, Xinhua)