KABUL, Afghanistan » Five American troops with a special operations unit were killed by a U.S. airstrike called in to help them after they were ambushed by the Taliban in southern Afghanistan, in one of the deadliest friendly fire incidents in nearly 14 years of war, officials said Tuesday.

The deaths were a fresh reminder that the conflict is nowhere near over for some U.S. troops, who will keep fighting for at least two more years.

Pentagon spokesman Rear Adm. John Kirby said the five American troops were killed Monday "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," Kirby said in a statement.

In Washington, U.S. defense officials said the five Americans were with a special operations unit that they did not identify. Earlier, officials had said all five were special operations-qualified troops, but later an official said their exact affiliation was unclear and one or more may have been a conventional soldier working with the special operations unit.

The deaths occurred during a joint operation of Afghan and NATO forces in the Arghandab district of southern Zabul province ahead of Saturday’s presidential runoff election, said provincial police chief Gen. Ghulam Sakhi Rooghlawanay. After the operation was over, the troops came under attack from the Taliban and called in air support, he said.

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

There was no way to independently confirm Rooghlawanay’s comments. The coalition would not comment and NATO headquarters in Brussels also declined to comment.

However, special operations forces often come under fire on joint operations and are responsible for calling in air support when needed. Because of constraints placed by President Hamid Karzai, such airstrikes are usually called "in extremis," when troops fear they are about to be killed.

Airstrikes have long caused tensions between the Afghan government and coalition forces, especially when they cause civilian casualties.

Airstrikes that kill coalition soldiers are far less common. One of the worst such incidents came in April 2002, when four Canadian soldiers were killed by an American F-16 jet fighter that dropped a bomb on a group of troops during a night firing exercise in southern Kandahar. In April 2004, former National Football League player Pat Tillman was killed by coalition fire while serving in an Army Ranger unit in one of the most highly publicized cases.

One of the five American troops killed Monday was identified as 19-year-old Aaron Toppen of Mokena, Ill., who had deployed to Afghanistan in March, a month after his father died, according to a family spokeswoman, Jennie Swartz. His family was suffering a "double hit" of grief, Toppen’s sister, Amanda Gralewski, told the Chicago Sun-Times.

The Taliban claimed responsibility for Monday’s ambush in Zabul.

A Taliban spokesman, Qari Yousef Ahmadi, said a battle took place between foreign troops and Taliban fighters in the Arghandab district, and a "huge number" of NATO soldiers were killed or wounded in the fighting. The Taliban often exaggerate their claims.

The insurgents have intensified attacks on Afghan and foreign forces ahead of Saturday’s presidential runoff, and officials are concerned there could be more violence around the time of the vote, although the first round in April passed relatively peacefully.

Of the 30,000 or so U.S. troops left in Afghanistan, special operations forces are among the only ones that are active on the battlefield, mentoring and advising Afghan commandos during raids.

An even smaller group that operates independently of the NATO coalition mandate, which expires at the end of the year, goes after high-value targets including the remnants of al-Qaida. Many of those special forces are likely to remain after the end of 2014, when foreign combat troops leave the country.

Although the U.S. has pledged 9,800 troops will remain until the end of 2016, a bilateral security agreement allowing them to do so has yet to be signed. The two candidates vying to succeed Karzai have said they will sign the deal.

Most of those troops will be training and advising the Afghan army and police, but a small counterterrorism force will still go after high value Jihadists still in the country.

The main opposition candidate, former foreign minister Abdullah Abdullah, has little love for the Taliban and is unlikely to stand in the way of such operations. The other contender, former finance minister and Karzai adviser Ashraf Ghani Ahmadzai, may be more reticent.

Separately, a NATO statement said a service member died Monday as a result of a non-battle injury in eastern Afghanistan.

The deaths bring to 36 the number of NATO soldiers killed so far this year in Afghanistan, with eight service members killed in June.

Casualties have been falling in the U.S.-led military coalition as its forces pull back to allow the Afghan army and police to fight the Taliban insurgency. All combat troops are scheduled to be withdrawn from the country by the end of this year.

Violence against Afghans, however, has continued unabated.

Insurgents attacked two vehicles carrying civilian de-miners in eastern Logar province, killing eight and wounding three, said provincial spokesman Din Mohammad Darwesh.

In eastern Ghazni province, insurgents kidnapped 33 university instructors who were travelling to Kabul for a seminar. Kandahar provincial spokesman Dawa Khan Menapal said the 33 were taken by a large group of insurgents and there was no word on their fate. He said all were from the southern province.

Quinn contributed from Cairo. Associated Press writers Robert Burns and Hope Yen in Washington contributed to this report.