WARNING, GRAPHIC: Video From Outside Kabul Airport Shows The Carnage In The Immediate Aftermath Of Bombings

At least two explosions near the main airport in Kabul resulted in "a number of US & civilian casualties" on Thursday, the Pentagon said, the worst-case scenario for U.S. efforts to evacuate Americans and Afghan allies from Afghanistan ahead of President Biden's August 31 deadline to withdraw.

"We can confirm that the explosion at the Abbey Gate was the result of a complex attack that resulted in a number of US & civilian casualties," Pentagon press secretary John Kirby said on Twitter. "We can also confirm at least one other explosion at or near the Baron Hotel, a short distance from Abbey Gate. We will continue to update."

Abbey Gate is one of four entrances to the airport, where thousands of people had gathered everyday since Kabul fell to the Taliban, desperate to gain entry and board flights out of the country.

Three sources told CBS News a suicide bomber was responsible for the explosion at Abbey Gate. A White House official told CBS News that Mr. Biden was being briefed on the situation in the White House Situation Room.

As the U.S. rushes to evacuate U.S. citizens and Afghans who helped American troops during the 20-year war in Afghanistan, as well as those at risk from the Taliban, Mr. Biden had been warning that the risk to American and allied forces on the ground was growing with each day that passes.

On Wednesday evening, the U.S. and Britain warned citizens not to go to Kabul's airport because of a terror threat outside the facility's gates. The U.S. Embassy in the capital alerted U.S. citizens to avoid traveling to the airport and said those at three different gates "should leave immediately."