(Recasts with suicide bombing, adds casualties)
WASHINGTON, Aug 26 (Reuters) - An explosion outside Kabul's airport amid a huge evacuation effort from Afghanistan appeared to be caused by a suicide bomb, U.S. officials said on Thursday, citing an initial report and cautioning that it could change.
Pentagon Press Secretary John Kirby confirmed in a Twitter post that there had been an explosion outside the airport in Afghanistan's capital.
A U.S. official told Reuters there were casualties from the explosion, but said it was unclear how many people were hurt. As many as three U.S. service members were among those injured, the official said, citing initial information.
Two U.S. officials said it appeared to be a suicide bombing.
A massive airlift of foreign nationals and their families as well as some Afghans has been under way since the day before Taliban forces captured Kabul on Aug. 15, capping a swift advance across the country as U.S. and allied troops withdrew.
President Joe Biden has been briefed on the explosion, according to a White House official. Biden was in a meeting with security officials about the situation in Afghanistan, where the United States is in the final steps of ending its 20-year war, when the explosion was first reported, according to a person familiar with the matter.
The United States has been racing to carry out the airlift before its military is set to fully withdraw from the country on Aug. 31.
In an alert issued on Wednesday, the U.S. Embassy in Kabul had advised citizens to avoid traveling to the airport and said those already at the gates should leave immediately, citing unspecified "security threats."
A Western diplomat in Kabul said that areas outside the airport gates had been "incredibly crowded" again despite the warnings.
The United States and its allies have mounted one of the biggest air evacuations in history, bringing out about 95,700 people, including 13,400 on Wednesday, the White House said on Thursday. (Reporting by Lisa Lambert, Idrees Ali, and Trevor Hunnicutt; editing by Will Dunham and John Stonestreet)
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