Separately, four Canadian soldiers and a journalist imbedded in their unit were killed Wednesday by a roadside bomb in the southern Kandahar province, the bloodiest single incident suffered by that country's military in 2009.
Michelle Lang, a 34-year-old health reporter with the Calgary Herald, was the first Canadian journalist to die in Afghanistan. She arrived in the country just two weeks ago.
The Taliban claimed responsibility for both attacks in messages to The Associated Press.
Also Thursday, a spokesman for the governor of Helmand province in the south said an airstrike by international forces killed and wounded civilians. Dawud Ahmadi said he did not have immediate information on how many were killed in the Wednesday incident in Babajid district, which he said occurred after an international forces patrol came under fire.
NATO said it was aware of the reports and was investigating. Claims of civilians killed by foreign forces are a highly emotional issue among Afghans and feed strong resentment of international soldiers.
It was not immediately clear how the suicide bomber at the American base at the edge of Khost city was able to circumvent security.
Taliban spokesman Zabiullah Mujahid said in a statement that an Afghan National Army officer wearing a suicide vest entered the Khost base Wednesday and blew himself up inside the gym. A U.S. official who was briefed on the blast also said it took place in the gym.
Khost is the capital of Khost province, which borders Pakistan and is a Taliban stronghold.
The U.S. official said eight American civilians and one Afghan were killed; it was not clear if the Afghan victim was military or civilian. Six Americans were wounded, the official said. The CIA has not yet commented on or confirmed the deaths.
There was no independent confirmation that the bomber was a member of the Afghan military. Gen. Mohammad Zahir Azimi, spokesman for the Afghan Ministry of Defense, said no Afghan National Army soldiers are at the base.
But an Afghan official in Khost said about 200 Afghans have been contracted by the U.S. to take care of security at the base. They are usually deployed on the outer ring of its walls, although some work inside, the official said on condition of anonymity because of the sensitivity of the issue.
"It's not the first time that Afghan forces have conducted such an attack to kill Americans or foreigners," the Taliban statement said, citing the killing of an American soldier and the wounding of two Italians this week in Badghis province. NATO has provided no details of that incident, but Afghan Gen. Jalander Shah Bahnam said an Afghan soldier opened fire on a base in the province's Bala Murghab district.
The congressional official in Washington said it was not clear how many of the victims in Khost were assigned to the CIA.
A former senior CIA 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.
All the U.S. officials spoke on condition of anonymity because they were not authorized to discuss the matter with the media.
NATO said only that the base is used by provincial reconstruction teams, which consist of both soldiers and civilians, and other personnel.
A spokesman in Kabul for the international coalition force said no U.S. or NATO troops were killed in the explosion. The attack was the bloodiest for Americans since eight soldiers were killed in an insurgent attack on a base in eastern Afghanistan on Oct. 3.
In the south, NATO said the four Canadian troops and the reporter died when their armored vehicle hit a bomb while on an afternoon patrol south of Kandahar city. It was the third-deadliest day for Canadians in Afghanistan since the war began.
Lang "was one of those journalists who always wanted to get to the bottom of every story so this was an important trip for her," said a Calgary Herald colleague, Colette Derworiz.
The military has not disclosed the names of the Canadian troops because relatives have not all been notified.
Brig. Gen. Daniel Menard, commander of coalition forces in Kandahar, said the soldiers were conducting a community security patrol.
Wednesday's attack was the second lethal strike against Canadian forces in a week. One Canadian soldier and an Afghan soldier were killed Dec. 23 during a foot patrol in Panjwayi district of Kandahar province. According to figures compiled by The Associated Press, the latest casualties bring to 32 the number of Canadian forces killed in Afghanistan this year; in all, 138 have died in the war.
Afghan President Hamid Karzai issued a statement of condolence to Americans and Canadians, saying "your children sacrificed their lives for the people of Afghanistan and the threat of terrorism. The Afghans will not forget your sacrifice."