The Pentagon confirmed Thursday that there was an explosion outside the Hamid Karzai International Airport in Kabul on Thursday.

The big picture: "Casualties are unclear at this time," Pentagon spokesperson John Kirby added in a tweet. The State Department issued a warning on Wednesday night urging Americans not to travel to the airport and to leave immediately if they were gathered near the Abbey Gate, East Gate, or North Gate.

• The U.K. and Australia gave similar warnings. James Heappey, the British armed forces minister, told the BBC that the threat of a suicide attack by ISIS forces was "credible, imminent and lethal."
• The White House has stressed for several days that the possibility of an attack from ISIS-K was one factor pushing them to attempt to complete the evacuation operation by Aug. 31. 

This is a developing story. Please check back for updates.