A Pentagon spokesperson has confirmed that an explosion outside Kabul's airport on Thursday "resulted in a number of US and civilian casualties."

US Defense Department Press Secretary John Kirby tweeted on Thursday that the blast occurred "near the Abbey Gate of the Kabul airport."

He added that "at least one other explosion at or near the Baron Hotel, a short distance from the Abbey Gate."

Reuters news agency quoted US officials as saying the blast near the Abbey Gate appeared to be caused by a suicide bomb.

The United States and Britain had been advising people to stay away from the airport, citing unspecified security threats.