Jim Michaels

USA TODAY

The death of five American troops in a suspected friendly fire airstrike in Afghanistan marks the deadliest such incident involving coalition forces during nearly 13 years of fighting and comes at a time when casualties from enemy fire are at the war's low point.

Monday's tragedy was disclosed Tuesday by the Pentagon, which has turned most combat operations over to Afghan security forces as the United States continues to reduce the number of troops there.

"We do have reason to suspect that friendly fire was the cause," Pentagon press secretary Rear Adm. John Kirby said. An Afghan soldier was also killed in the incident, Afghan police said.

The airstrike occurred during an engagement with enemy forces in Zabul province, which borders Pakistan's tribal areas.

The troops were elite special operation forces, according to a coalition official. He asked that his name be withheld because he is not authorized to speak about the incident.

The number of U.S. forces in Afghanistan has declined to 31,800 from a peak of about 100,000 several years ago. The U.S. force is scheduled to be trimmed to less than 10,000 next year if Afghan's government signs a security agreement with the United States.

The number of U.S. combat deaths in Afghanistan has fallen to 14 so far this year, not including the latest deaths, down from a peak of 439 in 2010, according to a USA TODAY analysis of Pentagon data.

Airstrikes have been controversial in Afghanistan. Outgoing Afghan President Hamid Karzai has frequently criticized the coalition for airstrikes that have caused civilian casualties.

In March, a NATO airstrike in Afghanistan's eastern Logar province killed five Afghan soldiers and wounded eight others.

In April 2002, four Canadian troops were killed by a U.S. airstrike.

The most well-known and controversial friendly fire death took place in 2004, when Army Ranger and former professional football player Pat Tillman and an Afghan national were killed in a battle near the Pakistan border.

The Pentagon initially announced that Tillman was killed by enemy fire, but a month later revealed that another group of coalition forces had mistaken Tillman's group for the enemy.

In this week's incident, provincial Police Chief Gen. Ghulam Sakhi Rooghlawanay told the Associated Press that after a joint operation by Afghan and NATO, "the joint forces came under the attack of insurgents, and then foreign forces called for air support."

"Unfortunately five NATO soldiers and one Afghan army officer were killed mistakenly by NATO airstrike," he said.

Pajhwok Afghan News reported that an interpreter also was killed. Taliban spokesman Qari Yusuf Ahmadi told Pajhwok that Taliban fighters were locked in a gunbattle with foreign troops when coalition planes bombed their own troops.

The deaths bring to 36 the number of NATO troops killed so far this year in Afghanistan, including eight in June.

About 50,000 coalition troops are stationed in Afghanistan, more than 32,000 of them Americans.

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Contributing: John Bacon and Paul Overberg