Two American service members were killed during an operation in Afghanistan on Friday, the U.S. and NATO forces said, providing no other details on the combat deaths.

The fatalities, which bring to four the number of U.S. soldiers killed so far this year in Afghanistan, underscore the difficulties in bringing peace to the war-wrecked country even as Washington has stepped up efforts to find a way to end the 17-year war, America's longest.

The U.S. and NATO Resolute Support mission said the names of the service members killed in action were being withheld until after notification of the next of kin, in accordance with U.S. Department of Defense policy. The statement also did not specify the location of the combat or say who the soldiers were fighting.

"The incident is under investigation and we have no additional information to provide," said Sgt. 1st Class Debra Richardson, a Resolute Support spokeswoman.

A Taliban statement later in the day said insurgents engaged in heavy fighting with Afghan and U.S. forces overnight in the northern city of Kunduz. Zabihullah Mujahid, the Taliban spokesman, said the fighting was still underway Friday; he claimed the insurgents had killed as many as three Americans and nine Afghan commandos.

The insurgents often exaggerate their battlefield claims and it was impossible to confirm whether the fighting Mujahid was referring to was the same combat in which the two U.S. service members were killed.

An Afghan lawmaker from Kunduz province, Abdul Wodood Payman, said there was heavy fighting overnight in the Kunduz neighborhood of Taluka, where jet fighters roared overhead and bombings could be heard. He had no additional information.

There are about 14,000 U.S. forces in Afghanistan, supporting embattled Afghan forces as they struggle on two fronts — facing a resurgent Taliban who now hold sway over almost half the country and also the Islamic State affiliate, which has sought to expand its footprint in Afghanistan even as its self-proclaimed "caliphate" has crumbled in Syria and Iraq.

In 2001, after the Sept. 11 attacks on New York and Washington, the U.S. invaded Afghanistan and ousted the ruling Taliban regime in a matter of weeks. But the Taliban subsequently regrouped while Washington shifted its attention to Iraq's Saddam Hussein, and by 2009, the war had become a stalemate.

The Pentagon has recently been developing plans to withdraw up to half of the American forces still in the country while at the same time stepping up efforts and having the U.S. negotiate with the Taliban.

U.S. envoy Zalmay Khalilzad, the Trump administration's main negotiator with the Taliban, concluded earlier this month a 13-day marathon session with leaders of the insurgent group.

Following the talks, held in Qatar where the Taliban maintain an office, Khalilzad said the two sides reached two "draft agreements" covering the withdrawal of U.S. troops and guarantees that Afghanistan would not revert to a haven for terrorists.

But he was unable to persuade the Taliban to launch talks with the Afghan government. The Taliban have consistently refused to talk with the government in Kabul, describing it as a U.S. puppet.

The two sides seem to be in agreement about the withdrawal of American forces, but divided over the timeline and whether a residual American force would remain.

Last year, 13 U.S. service members were killed in Afghanistan.

The Taliban have often targeted the city of Kunduz, the provincial capital of Kunduz province, most recently in a pre-dawn attack in February on an army base on the city's outskirts that killed 26 members of the Afghan security forces.

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Associated Press writer Maamoun Youssef in Cairo contributed to this report.