At least 3 US service members wounded in airport attack: Official

A U.S. official said that at least three U.S. military service members have been wounded in the explosion at the Abbey Gate outside Hamid Karzai International Airport. The official did not know the extent of the injuries. U.S. troops have been stationed near large crowds trying to get inside.

The development comes as the U.S. Embassy in Kabul sent another security alert to Americans, warning, "There has been a large explosion at the airport, and there are reports of gunfire."

"U.S. citizens should avoid traveling to the airport and avoid airport gates at this time. U.S. citizens who are at the Abbey Gate, East Gate, or North Gate now should leave immediately," it said.

ABC News Chief Global Affairs Correspondent Martha Raddatz said the explosion at the airport coming days before U.S. forces are set to leave the country has made for, "truly, a nightmare scenario."

Raddatz, who is in contact with Afghans on the ground, said the explosion "devastates" the evacuation process and described the tone in the country as people try to get out in the coming days as "absolute panic."

"Now today, they're facing, either a stampede, the Taliban or ISIS bombers -- and that really is what it comes down to for all Afghans," she said.

A U.S. official confirms the explosion was at Abbey Gate -- one of the main entrances to the airport -- and that they are assessing casualty numbers right now.

A White House official told ABC News that  President Biden has been briefed on the situation.