A U.S. Navy SEAL was killed Tuesday in an attack by Islamic State fighters in northern Iraq, highlighting the evolving nature of the Pentagon’s mission in Iraq and how American troops are serving closer than ever to the front lines.

The SEAL, whose identity was withheld until family members were notified, was killed by enemy fire about 9:30 a.m., U.S. military officials said, speaking on the condition of anonymity because they were not authorized to release information publicly.

The death occurred after Islamic State fighters north of Mosul penetrated a front line of Kurdish peshmerga forces by about three miles, a U.S. military official said.

The SEAL was the third U.S. service member killed in combat since the U.S. military campaign against the Islamic State began in June 2014.

The first, Army Master Sgt. Joshua L. Wheeler, 39, was a member of the elite Delta Force who was killed in a raid Oct. 22. But the latest two deaths show the kind of threats faced by the bulk of U.S. troops advising Iraqi soldiers near the front lines with the Islamic State.

A Kurdish official said the death occurred after Islamic State fighters began attacking peshmerga lines at dawn near the town of Telskuf, about 20 miles north of Mosul, the Islamic State’s main stronghold in Iraq.

The attack involved “truck bombs supported by infantry,” the official added, an indication that common conventional Islamic State tactics were likely used.

An established front line – called a forward line of troops, or FLOT, by U.S. service members – has separated the Islamic State and Iraqi soldiers for months, and the Islamic State often tries to breach it using armored vehicles carrying explosives, with combat troops following behind.

Mortar rounds and artillery began hitting front lines near Telskuf, the largely Christian town, around 4 a.m., according to Kurdish officers and members of the Christian militia that hold the ground there.

Its inhabitants, speakers of ancient Aramaic, fled in August 2014 when the town was overrun by Islamic State forces, who burned and desecrated its churches. It was retaken by Kurdish forces later the same month, and some residents had since returned.

But after bombarding the area on Tuesday, the militants launched a multi-pronged attack on Telskuf at around 5:30 a.m. from three or four directions, using hundreds of fighters, commanders said.

Maj. Gen. Azad Jalil, a peshmerga officer, said they breached Kurdish front lines with more than 10 car bombs, also using bulldozers to push through.

The peshmerga then made a “tactical retreat” to reorganize their forces, he said. Islamic State militants overran the village.

Brig. Gen. Bahnam Aboush, a fighter with the Christian militia based in the town and known as the Nineveh Plain Protection Units, said his men tried to hold their ground but were overwhelmed.

“We tried to fight them, but we couldn’t due our limited capabilities,” he said. “We have only some old rifles we bought from our own money.”

He said he witnessed the attack that mortally wounded the Navy SEAL, when a U.S. military contingent came to assist their struggling forces.

“American special forces came to rescue us in four vehicles,” he said. “They opened the way for us to retreat, then one of their vehicles was hit” with a rocket-propelled grenade.

He said that one U.S. service member was seriously injured and was airlifted out by helicopter. “I heard he died after,” he said.

The Kurdish official, speaking on the condition of anonymity because of the sensitivity of the information, also said that the U.S. service member died as he was being transported out of the area.

In Washington, White House spokesman Josh Earnest said: “Today’s incident is a vivid reminder of the risk our service members are taking, and three of them now have made the ultimate sacrifice for our country.” He stressed that the U.S. mission in Iraq “is to support Iraqi forces on the ground that are taking the fight” to the Islamic State and that “Iraqi forces must fight for their own country.”

Earnest said that U.S. troops cannot act “as a substitute” for Iraqi forces.

After the Christian and Kurdish forces retreated from Telskuf, Islamic State militants took control of the town, until an offensive was launched to recapture it with the help of U.S. air power, commanders said. They had managed to regain control of the town by late afternoon on Tuesday.

“The coalition had the key role in retaking the village,” said Jalil. He said the Islamic State launched the attack because it is under pressure south of Mosul near Makhmour, where Iraqi forces, with the help of U.S. artillery and airpower, have managed to make some inching gains.

“It’s showing how desperate and how broken they are,” he said. “There are now more than 100 dead bodies of them in the village, plus they lost many vehicles,” he said.

Jabbar Yawar, a spokesman for the Kurdish peshmerga, said a number of Kurdish fighters died in the attack but that he could not disclose how many.

“This sad news is a reminder of the dangers our men and women in uniform face everyday in the ongoing fight to destroy ISIL and end the threat the group poses to the United States and the rest of the world,” Pentagon press secretary Peter Cook said in a statement, using an acronym for the Islamic State.

Defense Secretary Ashton B. Carter first announced the death while traveling in Stuttgart, Germany. He said the service member was killed by enemy fire but offered few additional details.

U.S. Army Special Operations troops operate across peshmerga front lines, often spending hours at outposts gathering information about the Islamic State’s activity.

The small detachments, however, are usually stationed a few miles from the front to help coordinate airstrikes between peshmerga fighters and the joint command centers in Baghdad and Irbil, the administrative center of the semiautonomous Kurdish region in northern Iraq.

Some 200 U.S. Marines are also now stationed less than 10 miles from the front line, near the northern town of Makhmour, where Iraqi troops are building up for a future Mosul offensive. Marine Staff Sgt. Louis Cardin was killed there in a March 19 rocket attack.

Prior to Tuesday, 15 American service members had been wounded in the campaign in addition to the three deaths, according to Pentagon statistics.

(AP)