The Islamic State (ISIS/ISIL) branch in Afghanistan, known as the Khorasan Province (IS-KP/ISIL-K), has claimed responsibility for planting the Improvised Explosive Device (IED) that killed a U.S. service member, bringing the number of American military fatalities during the Afghan war to 2,241.

Three Afghan soldiers were also killed by the explosion, which took place in eastern Afghanistan’s Nangarhar province, ISIL-K’s stronghold in the Afghanistan-Pakistan region.

An IED, also known as a homemade bomb, has been deemed the deadliest weapon terrorists use against the U.S.-led coalition and Afghan troops in the course of the ongoing 15-year-old war.

U.S. troops have been backing an ongoing offensive launched in July by the Afghan National Defense and Security Forces (ANDSF) against ISIL-K jihadists in Nangarhar, which lies along Afghanistan’s border with Pakistan.

The American service member, who has not been identified yet, was killed Tuesday while carrying out operations in Nangarhar. The Islamic State claimed responsibility for the attack the following day, according the jihadist group’s Amaq news agency:

#ISIS‘ ‘Amaq reported that 1 American & 3 Afghan soldiers were killed in an IED blast in Achin district in Nangarhar #Afghanistan on Oct 4 pic.twitter.com/rSeeikp2RR — SITE Intel Group (@siteintelgroup) October 5, 2016

Reuters reports:

Islamic State has attracted hundreds, perhaps thousands, of fighters in Afghanistan and Pakistan to join its ranks, and holds some territory in Nangarhar. But it has not been able to expand its influence in Afghanistan beyond a few districts and the Taliban remain the dominant militant force there.

Gen. John Nicholson, the top commander of U.S. and NATO forces in Afghanistan, recently told Pentagon reporters that about “1,200 to 1,300” ISIL-K members remain in Afghanistan, down from a peak of nearly 3,000.

At the end of last month, U.S. Forces in Afghanistan (USFOR-A) released a statement saying ISIL-K in Nangarhar was under pressure, using civilians as shields and “dressing in female attire” to escape the U.S.-backed counterterrorism offensive in the province.

In announcing the recent fatality, Peter Cook, the Pentagon’s press secretary, told reporters Tuesday:

I wanted to begin by expressing the sincere condolences not only from Secretary Carter, but from all of us within the Department of Defense to the family of the service member. We are still gathering details on exactly what happened, but we understand that this service member died as a result of injuries sustained from an improvised explosive device while the individual was partnered with Afghan forces in Nangarhar province in Afghanistan. The mission was conducted as a part of a larger U.S.-Afghan counterterrorism mission targeting ISIL in Afghanistan. …Our operations against ISIL in Afghanistan continue alongside our Afghan partners.

The Obama administration has been reluctant to say the U.S. military in Afghanistan is still engaged in combat.

Cook refused to explicitly say the U.S. service member was killed in combat on Tuesday, arguing, instead, that the service member was involved in a “combat situation.”

The spokesman said:

We are targeting ISIL in Afghanistan. We have soldiers in harm’s way. This clearly was a tragic situation. It highlights the risks that our service members are taking every single day in Afghanistan and elsewhere, and we’ve been quite clear about that. … This was a combat situation.

President Barack Obama declared the U.S. combat mission complete at the end of 2014, a move that was supposed to mark the beginning of the train, assist, and advise (TAA) role for American troops.

However, U.S. troops have since been drawn into combat against a resilient Taliban and its rival ISIL-K.

Nearly a year after Obama announced the end of the U.S. combat mission, he granted authority to the U.S. military to take offensive action against ISIL-K.

Earlier this year, Obama also renewed the U.S. military’s authority to offensively attack the Taliban.

Since the president declared an end to the U.S. combat mission in Afghanistan, 124 American service members have been wounded in action in the country, and 25 others have died, including 12 who were killed in action.

Overall, 2,241 U.S. military troops have been killed during the ongoing Afghanistan war, and 20,173 others have been injured. U.S. military fatalities have quadrupled under Obama’s watch, and injuries have increased more than sevenfold.