Members of the U.S. 82nd Combat Aviation Brigade salute during a ceremony to transfer authority to American command in May at Kandahar Airfield in Afghanistan. More than 300 American troops have died in Afghanistan this year. ((Patrice Bergeron/Canadian Press))

Eight Americans were killed in an apparent suicide attack in southeastern Afghanistan on Wednesday, U.S. officials said.

CIA employees are believed to be among the dead. A former senior Central Intelligence Agency officer who was stationed at the base said a combination of agency officers and contractors operated out of the remote outpost with the military and other agencies. The CIA has not commented.

The Americans were killed by an attacker wearing a suicide vest at Forward Operating Base Chapman in Khost province near the Afghanistan-Pakistan border, according to a senior official in Washington who spoke on condition of anonymity because not all the details have been confirmed.

State Department spokesman Ian Kelly did confirm that eight Americans were killed.

"We mourn the loss of life in this attack, and are withholding further details pending notification of next of kin," he said.

There were conflicting reports on the total number of casualties, but one newspaper said several people were injured, some seriously.

Wazir Pacha, a police spokesman in Khost province, said local people reported hearing the blast at a military base in the area.

A total of 311 American troops have died in Afghanistan this year, more than double the 150 U.S. service members killed in Iraq during the same period.

With files from The Associated Press