Four Americans were killed Saturday in an attack at a U.S. airfield in Afghanistan carried out by a suicide bomber.

Defense Secretary Ash Carter said the explosion inside Bagram Airfield took the lives of two U.S. servicemembers and two U.S. contractors working on the base and wounded 16 other U.S. servicemembers and one Police solider.

“I want to express my sincere condolences to the families of the fallen, and I want to reassure the loved ones of those injured that they are getting the best possible care,” Carter said.

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The State Department announced the U.S. Embassy in Kabul would close Sunday as a precaution.

An earlier statement from NATO's Resolute Support mission said the blast happened around 5:30 a.m. (8 p.m. Friday EST) and that "force protection and medical teams are responding to the situation."

The Taliban claimed responsibility for the attack. Zabihullah Mujahid, a spokesman for the insurgent group, said the attack had been planned for four months.

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The Taliban regularly fire rockets at Bagram from outside its perimeter.

Abdul Wahid Sediqqi, spokesman for the governor of Parwan province, where the air field is based, said laborers employed at the base line up at the gates around dawn. He said it was possible the attacker could have been among the men entering the base Saturday.

The base houses the largest contingent of American troops and contractors in Afghanistan.  

The explosion follows an attack on German consulate Thursday in northern Afghanistan, Fox News reports.  

Saturday's attack brings to 11 the number of Americans, including two CIA paramilitary officers, who have been killed in action in Afghanistan since early October. 

The US military killed two top al-Qaeda leaders in eastern Afghanistan in late October, the Pentagon says they were plotting new attacks against the United States more than 15 years since the 9/11 attacks. 

The Taliban have made gains in recent months. The top US commander in Afghanistan said over a month ago the Taliban control or contest up to a quarter of country. 

Fox News' Lucas Tomlinson and the Associated Press contributed to this report.