A U.S. service member was killed and four others were wounded Monday during an operation in a part of Afghanistan in which U.S. forces have been combating the Islamic State and the Taliban, U.S. military officials said Tuesday.

The fatality occurred in Nangahar province’s Achin district, in a mountainous area along the border with Pakistan. The Islamic State in Khorasan, the militant group’s Afghan affiliate, established roots there in 2014, and the U.S. military launched raids and airstrikes there in 2017 as it stepped up its campaign against the group.

U.S. military officials acknowledged the fatality Tuesday in a news release, saying two of the wounded service members were receiving treatment at a nearby medical facility and were in stable condition. The other two wounded service members have returned to duty, the news release said.

“We are deeply saddened by the loss of one of our own,” Army Gen. John W. Nicholson Jr., the senior U.S. military commander in Afghanistan, said. “At this very difficult time our heartfelt sympathies go out to the families and friends of our fallen and wounded brothers.”

U.S. military officials in Afghanistan declined to release additional information about the incident, and the name of the service member killed was being withheld as family members were notified.

Sen. Cory Booker, D.-New Jersey, identified the soldier as Army Sgt. 1st Class Mihail Golin, of Fort Lee, New Jersey, outside New York City.

“Sergeant First Class Mihail Golin served our nation with courage and distinction, and his death is a loss that will be felt across New Jersey,” Booker said. “Let us honor Sergeant Golin’s extraordinary courage by reflecting on his commitment to the nation he loved, and by recognizing the profound debt of gratitude we owe to him and to his family for their sacrifice.”

Navy Capt. Tom Gresback, a U.S. military spokesman in Kabul, said Tuesday that “there has been a tremendous amount of kinetic activity in the Achin district” of late, using a military term that typically connotes firefights and other violence.

“We continue to work very closely with our Afghan partners in aggressively clearing significant amount of territory in Southern Nangarhar,” Gresback said in an email. “We continue to apply increased military pressure on ISIS-K, forcing their movement from their front-line positions.”

Gresback said U.S. and Afghan forces are making “significant progress” in the area, and predicted a long winter for militants in the region.

At least 15 U.S. service members died in Afghanistan in 2017, including 11 in hostile actions. Seven of the 11 killed in combat died in Nangahar province. The most recent combat fatality there occurred Aug. 16, when Army Staff Sgt. Aaron R. Butler, 27, was killed by an improvised device. He was a member of the 19th Special Forces Group.

Nicholson told Pentagon reporters in a news briefing Nov. 28 that ISIS-K at one point had a presence in nine districts spanning three provinces in eastern Afghanistan, but that U.S.-led counterterrorism operations had reduced that to three. Militants in Achin were pushed into mountains that border Pakistan and were attempting to move west from there through access points in the mountains, the general said.

“Remember most of these Daesh fighters came from Pakistan,” Nicholson said, using an alternate name for ISIS. “They go through the passes of southern Nangahar and they move back to their home agency.”

The general said that there were likely about 600 to 800 ISIS fighters in Nangahar. An additional 300 were spread out between two other provinces, in his estimation: Kunar province, which borders Nangahar, and Jowzjan, in northwestern Afghanistan.

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