KABUL — Five American troops were killed in a possible "friendly fire" incident in southern Afghanistan on Monday, in what was the deadliest day for the international military coalition there in six months.

A Pentagon statement said the troops died during a security operation in southern Afghanistan. “Investigators are looking into the likelihood that friendly fire was the cause,” it said.

According to the Associated Press, two U.S. defense officials say the five Americans killed were special operations troops.

Zabul police chief Ghulam Sakhi Roghlewani said the Americans were killed after being ambushed by insurgents in Zabul province. Roghlewani said an Afghan soldier also died after coalition troops called in air support and an aircraft mistakenly struck their position. An official with Afghanistan’s intelligence agency, the National Directorate of Security, confirmed the details of the attack but said that two Afghan soldiers had been killed.

Another coalition servicemember died of “nonbattle-related” injuries in a separate incident in eastern Afghanistan.

An ISAF statement confirmed “the possibility that fratricide may have been involved” in the incident in Zabul province, adding that it was under investigation.

As the international combat mission in Afghanistan winds down, Afghan security forces have taken over most of the day-to-day fighting in the country, and coalition troop deaths have plummeted. The summer months, however, are traditionally the deadliest, as insurgents stream back into the country from their winter redoubts in Pakistan.

The last time this many troops died in one day was Dec. 17, when six American soldiers were killed after insurgents brought down their Black Hawk helicopter, according to, a site that tracks troop casualties in Afghanistan. Incidents of "friendly fire" have been rare in recent years, as ISAF has severely restricted its aerial bombing and tightened rules on close air support, partly because of outrage over civilian casualties.

If confirmed that "friendly fire" caused Monday’s incident, it would be the deadliest instance of fratricide since 2010.

German troops accidentally killed six Afghan soldiers in April 2010 while traveling to the scene of a clash with the Taliban in northern Afghanistan. In 2002, in what is known as the Tarnak Farms incident, four Canadian soldiers were killed and eight others were injured when an American F-16 pilot mistakenly dropped a bomb on a training range near the southern city of Kandahar.

Although "friendly fire" incidents have been common in all wars, the military says they have declined sharply since the start of the conflicts in Iraq and Afghanistan. Technological developments since the 1990s have greatly improved the ability of commanders to track friendly forces in combat and thus avoid identification failures, a leading cause of fratricide. Still, technological improvements can make "friendly fire" incidents more lethal than in the past. The very powerful weaponry now commonly used in combat can mean more casualties resulting from any single mistake, as in the case of Tarnak Farms.

Air strikes have long been a point of contention between the West and Kabul, with Afghan President Hamid Karzai frequently criticizing ISAF for civilian casualties caused by the attacks and issuing an executive order banning such strikes in populated areas. Karzai also banned Afghan troops from requesting foreign air support.

In response to such criticism and fewer coalition operations, ISAF has reduced the use of close air support in recent years. This worries some in the Afghan military, which still lacks the capability to provide air cover for troops on the ground.

According to a UN report in February, civilian deaths from coalition air strikes dropped by 10 percent between 2012 and 2013.

More than 3,400 foreign troops have been killed supporting the military operation in Afghanistan since October 2001, including more than 2,300 Americans.

Zubair Babakarkhail contributed to this report. Twitter: @Druzin_Stripes

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