– All six of the U.S. soldiers who have died in combat in Afghanistan this year were Special Operations troops involved in the fight against ISIS in its stronghold in a small eastern area of the country.

Five of the six may have been killed by their own side, according to reports from U.S. and Afghan military officials.

The casualty rate shows a stepped-up concentration on fighting the local branch of the extremist group, known here as the Islamic State in Khorasan, which two years ago was dismissed as a small breakaway faction, numbering in the low hundreds, of the much more powerful Taliban.

Since March, the U.S. military has said that joint Afghan-U.S. forces have killed or captured hundreds of ISIS fighters.

Last year, one U.S. soldier was killed in combat in Nangarhar Province, the eastern area that includes the Achin District — ISIS’ stronghold — in mountains close to the Pakistani border. In all, nine U.S. soldiers were killed in 2016, four of them Special Operations forces in combat roles. The others were on bases or in support roles.

The deaths on Saturday of three U.S. Special Operations soldiers — a fourth was wounded and transported out of Afghanistan — were the result of a “green on blue,” or insider, attack by an Afghan commando, said Jawed Salim, a spokesman for the Afghan Army Special Forces command.

But Afghan officials were skeptical of a claim by the Taliban that the commando had infiltrated the unit intending to carry out the attack.

“It is part of their propaganda war,” said Gen. Mohammad Radmanish, a spokesman for the Ministry of Defense. “We need to investigate what really happened here.”

The attacker was killed by the Americans, who returned fire, Afghan officials said.

The U.S. military has not yet commented in any detail on Saturday’s attack, and the victims’ names have not been released pending notification of next of kin. The military rarely releases Special Operations details.

Although the U.S. military is ostensibly on an “advise and assist” mission to aid Afghan forces, the Obama and Trump administrations have granted wide latitude for Special Operations forces to participate in Afghan combat missions.

Afghan officials confirmed that the latest deaths took place in the midst of an operation against ISIS, despite the Taliban’s claim of responsibility.

Vice President Mike Pence, speaking at an event in Milwaukee on Saturday, said that he and President Donald Trump had been briefed on the latest deaths.

“When heroes fall, Americans grieve, and our thoughts and prayers are with the families of these American heroes,” Pence said.

The first American killed in Afghanistan this year, Staff Sgt. Mark R. De Alencar, 37, of Edgewood, Md., was also involved in an operation against ISIS in Nangarhar Province, according to a statement from the Defense Department.

A much-decorated veteran of the Army in Iraq, the Special Forces sergeant gained his Green Beret in 2016, according to Military Times. He “died April 8 in Nangarhar Province, Afghanistan, of injuries suffered when his unit came in contact with enemy forces using small-arms fire during combat operations,” the Defense Department said.

Just over two weeks later, on April 26, two Army Rangers were killed in Achin District during a major operation against ISIS. Pentagon spokesman Capt. Jeff Davis said they might have been killed by friendly fire, presumably from close-air support during the fight.

“We are investigating the circumstances of the combat deaths of the two Army Rangers in the beginning of what was an intense three-hour firefight,” Davis said.

The military identified the men as Sgt. Joshua P. Rodgers, 22, of Bloomington, Ill., and Sgt. Cameron H. Thomas, 23, of Kettering, Ohio.

Ten days later, the U.S. military commander in Afghanistan, Gen. John W. Nicholson Jr., said that the operation had also killed the “emir” of the local Islamic State in Khorasan, Sheikh Abdul Hasib.

“This successful joint operation is another important step in our relentless campaign to defeat ISIS-K in 2017,” Nicholson said, using the military’s shorthand for the local ISIS affiliate. “This is the second ISIS-K emir we have killed in nine months, along with dozens of their leaders and hundreds of their fighters.”

The U.S. commander said that Hasib had been responsible for a deadly attack in March on a military hospital in Kabul in which militants killed more than 50 people, many of them patients.

Between the two fatal episodes in April, the U.S. military dropped a GBU-43/B Massive Ordnance Air Blast bomb, known as the Mother of All Bombs, the largest non-nuclear bomb ever deployed, on what it said was an ISIS tunnel complex in Achin District.

It was unclear how much damage the 22,000-pound bomb caused, and Afghan officials offered no proof for their assertion that 96 militants had been killed and that buildings had been destroyed in a 2-mile radius. ISIS radio had disputed the claims.

The three Special Operations soldiers killed on Saturday were fighting about a mile from where the MOAB bomb had been dropped, Afghan officials said.