Taliban spokesman Zabiullah Mujahid told The Associated Press that a Taliban bomber wearing a military uniform and a suicide vest entered the base in Khost province Wednesday evening 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.
That official said eight U.S. 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.
The congressional official in Washington said it was not clear how many of the victims were assigned to the CIA. The official was not authorized to discuss the matter with the media, and spoke on condition of anonymity.
A senior State Department official said all of the victims were civilians. 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. He said contractors also might be among those who died.
The CIA has not commented or confirmed any deaths.
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.
In Kabul, a spokesman for the international coalition force in Kabul said no U.S. or NATO troops were killed in the afternoon explosion. The attack was the bloodiest day 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 that the four Canadian troops and a reporter embedded with their unit died when their armored vehicle hit a bomb while on an afternoon patrol south of Kandahar city.
It's the third deadliest day for Canadians in Afghanistan since the war began.
Michelle Lang, a 34-year-old health reporter with the Calgary Herald, was the first Canadian journalist to die in Afghanistan. Lang arrived in Afghanistan just two weeks ago.
"She 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 her Calgary Herald colleague, Colette Derworiz.
The military has not disclosed the names of the Canadian troops because relatives had not all been notified.
"We are all very saddened to hear this tragic news," Alberta Health and Wellness Minister Ron Liepert said in a statement. "Michelle covered health issues with professionalism, accuracy and thoroughness. She was tenacious in her quest to inform Albertans, and for her diligence she was very well respected."
Brig. Gen. Daniel Menard, commander of coalition forces in Kandahar, said that the soldiers were conducting a community security patrol in order to gather information about daily life in the area and how to maintain security.
Wednesday was the second lethal strike against the Canadian force 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.
Separately on Wednesday, NATO questioned Afghan reports that international troops killed 10 civilians, including schoolchildren, in a weekend attack that prompted hundreds of angry Afghan protesters to burn an effigy of U.S. President Barack Obama and chant "death" to America.
The head of an investigative team appointed by Afghan President Hamid Karzai told The Associated Press by telephone that eight students between the ages of 12 and 14 were among the dead discovered in a village house in a remote section of Kunar province in eastern Afghanistan. NATO said in a statement released late Wednesday night that while there was no direct evidence to substantiate the claims, the international force had requested and welcomed a joint investigation to reach an "impartial and accurate determination" of what happened in the attack.
Conflicting accounts of what occurred during fighting in Kunar's Narang district prompted an emotional outcry over civilian deaths, one of the most sensitive issues for international troops fighting the more than eight-year-old war. Although insurgents are responsible for the deaths of far more civilians, those blamed on coalition forces spark the most resentment and undermine the fight against militants. With 37,000 more U.S. and NATO troops being deployed to the battle zone, concern over civilian casualties is unlikely to ease anytime soon.
Several hundred Afghans demonstrated in the capital of Kabul and in the eastern city of Jalalabad where the likeness of Obama, adorned with a small American flag, burned on a pole held above demonstrators.
Associated Press Writers Deb Riechmann in Kabul, Matthew Lee, Pam Hess and Pauline Jelinek in Washington and Adam Goldman in New York City contributed to this report.