Baghdad — 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 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.