CIA Base Chief Killed In Afghan Attack: AP

The CIA did not release information about the victims, citing the sensitivity of their mission and other ongoing operations.

According to one former agency employee, the death toll represents a significant portion of the CIA's clandestine force in the region, but is unlikely to cripple the agency because so many of its employees have experience in Afghanistan.

"The bench is deeper in Afghanistan than it is anywhere in the world," the former employee said.

The bigger question for CIA operations will probably be whether the agency moves to tighten safety rules for its employees, the former employee said.

The incident occurred at a former military base on the edge of Khost city, the capital of Khost province which borders Pakistan and is a Taliban stronghold.

The Taliban claimed responsibility.

Taliban spokesman Zabiullah Mujahid said in a statement that an Afghan National Army officer wearing a suicide vest entered the base and blew himself up inside the gym. A U.S. official briefed on the blast also said it took place in the gym.

Forward Operating Base Chapman used to be a military facility base but was later turned into a CIA base, according to a U.S. official, who spoke on condition of anonymity because he was not authorized to speak publicly.

Only four known CIA operatives have been killed in Afghanistan since the 9/11 terrorist attacks in the U.S.

CIA officer Micheal "Mike" Spann was killed in a prison uprising in November 2001. An agency officer died in a training exercise in 2003, and two contractors operating out of a CIA base in Shkin district of Paktika province were killed the same year.

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Adam Goldman reported from New York. Associated Press writers Matthew Lee and Anne Flaherty contributed to this report.