Five U.S. service members died in southern Afghanistan in a possible case of friendly fire. Afghan media are citing a local official who says the troops' air support mistakenly bombed their position. The attack is still under investigation.

Update at 2:45 p.m. ET: More From Pentagon

"We have reason to suspect that friendly fire was the cause here, specifically from the air," Pentagon spokesman Rear Adm. John Kirby said without elaborating.

"This is a tragic incident all around and our thoughts and prayers go out to the families," Kirby said.

Update at 10:30 a.m. ET: Pentagon Statement

"Five American troops were killed yesterday during a security operation in southern Afghanistan. Investigators are looking into the likelihood that friendly fire was the cause. Our thoughts and prayers are with the families of these fallen," says Pentagon spokesman Rear Adm. John Kirby .

As CNN reminds us , there have been several incidents of friendly fire during the Afghanistan operation, including in 2007, when three British soldiers died after a U.S. jet bombed their position.

From Kabul, NPR's Sean Carberry reports:

"Afghan officials claim that the five American troops and one Afghan Army soldier were killed by a NATO airstrike in southern Zabul province. NATO would not provide outright confirmation but in a statement said that fratricide might have been involved and the incident is under investigation."Officials say the U.S. and Afghan forces were conducting a joint operation to clear out militants in advance of Saturday's presidential election. While returning to their base, the forces came under attack and called in the air support."Counting these deaths, there have now been 40 NATO service members killed in Afghanistan this year. More than 40 percent of them died as a result of non-battle-related injuries."

The International Security Assistance Force released its statement about the deaths today.

The Pajhwok Afghan news agency cites an official in Zabul who "said ISAF aircrafts mistakenly bombed international troops after they called in air support during an operation in Gizee area of Arghandab district."

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