WASHINGTON - A suicide bomber infiltrated a CIA base in eastern Afghanistan yesterday, killing at least eight Americans in what is believed to be the deadliest single attack on U.S. intelligence personnel in the eight-year war and one of the deadliest in the agency's history, U.S. officials said.

The attack represented an audacious blow to intelligence operatives at the vanguard of U.S. counterterrorism operations in both Afghanistan and Pakistan, killing officials whose job involves plotting strikes against the Taliban, al-Qaeda, and other extremist groups that are active on the frontier between the two nations. The facility that was targeted - Forward Operating Base Chapman - is in the eastern Afghan province of Khost, which borders North Waziristan, the Pakistani tribal area that is believed to be al-Qaeda's home base.

U.S. sources confirmed that all the dead and injured were civilians and said they believed that most, if not all, were CIA employees or contractors. At least one Afghan civilian also was killed, the sources said.

It is unclear how the assailant managed to gain access to the heavily guarded U.S.-run post, which serves as an operations and surveillance center for the CIA. The bomber struck in what one U.S. official described as the base's fitness center.

In addition to the dead, eight people were wounded, several seriously, U.S. government officials said.

Also yesterday, NATO announced that four Canadian troops and a journalist from Canada were killed in an explosion in Kandahar province, one of the most dangerous areas of southern Afghanistan. The international coalition said the journalist was traveling with the troops on patrol.

While many details of the attack on the CIA base remained vague, the attack appears to have killed more U.S. intelligence personnel than have died in Afghanistan since the U.S.-led invasion began in late 2001. The CIA has previously acknowledged the deaths of four officers in fighting in Afghanistan in the last eight years.

"It is the nightmare we've been anticipating since we went into Afghanistan and Iraq," said John McLaughlin, a former CIA deputy director who now serves on a board that supports children of CIA officers slain on the job. "Our people are often out on the front line, without adequate force protection, and they put their lives quite literally in jeopardy."

The CIA has declined to comment publicly on the attack until relatives of the dead are notified. A former senior agency official said it was the worst single-day casualty toll for the agency since eight CIA officers were killed in the 1983 attack on the U.S. Embassy in Beirut, Lebanon.

The CIA has been quietly bolstering its ranks in Afghanistan in recent weeks, mirroring the surge of military troops there. Agency officers coordinated the initial U.S.-led attack against the Taliban in Afghanistan in late 2001 and have since provided hundreds of spies, paramilitary operatives, and analysts in the region for roles ranging from counterterrorism to counter-narcotics.

The agency also operates the remote-control aircraft used in aerial strikes on suspected al-Qaeda and Taliban fighters in the lawless tribal provinces on the Pakistan side of the border.

Intelligence experts who have visited U.S. bases in the region say the CIA officers at Chapman would have focused mainly on recruiting local operatives and identifying targets.

"The best intelligence is going to come from the field, and that means working closely with the Afghans," said Bruce Hoffman, a terrorism expert and professor at Georgetown University's School of Foreign Service.

The loss of experienced CIA field officers would be particularly damaging to U.S. efforts in the area "because they know the terrain," Hoffman said. "Every American death in a theater of war is tragic, but these might be more consequential given these officers' unique capabilities and attributes."

The bomber and those who aided him must have had very good intelligence to gain access to the secure base without arousing suspicion, he said.

U.S. military officials and diplomats confirmed yesterday's attack and the eight civilian deaths. "We mourn the loss of life in this attack," State Department spokesman Ian Kelly said.

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.

The Canadian Press identified the journalist as Michelle Lang, 34, a health reporter with the Calgary Herald, who was on her first assignment to Afghanistan. She was recently engaged and planned to marry next summer, according to the newspaper.

"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."

Dennis Skulsky, president and chief executive officer of Canwest Publishing, said in a statement on the company's Web site: "This is an unbelievable loss to her fiance, Michael, her family and the countless colleagues and friends at the Calgary Herald, Canwest News Service and across our organization."

Brig. Gen. Daniel Menard, commander of coalition forces in Kandahar, told Canadian Press early today that the soldiers were conducting a community security patrol to gather information about daily life in the area and how to maintain security.

This article contains information from the Associated Press.