Iraqi army soldiers stand outside a house on the edge of Kharbadan, Iraq, on April 19. Iraqi troops have recaptured a cluster of hamlets and villages in the vicinity of Makhmour, though reports were mixed on how heavy the Islamic State presence was there before the Iraqi advance. MUST CREDIT: photo for The Washington Post by Alice Martins.
Iraqi army soldiers stand outside a house on the edge of Kharbadan, Iraq, on April 19. Iraqi troops have recaptured a cluster of hamlets and villages in the vicinity of Makhmour, though reports were mixed on how heavy the Islamic State presence was there before the Iraqi advance. MUST CREDIT: photo for The Washington Post by Alice Martins.

Makhmour, Iraq — At the base of a rocky ridge rising from the surrounding farmland, the barrels of American artillery poke out from under camouflage covers, their sights trained on Islamic State-held positions.

Less than 10 miles from the front lines in the push toward the northern Iraqi city of Mosul, the U.S. outpost, known as Firebase Bell, is manned by about 200 Marines.

“Having them here has raised the morale of our fighters,” said Lt. Col. Helan Mahmood, the head of a commando regiment in the Iraqi army, as his truck bumped along the dirt track that divides his base from the American encampment, ringed by razor wire and berms.

“If there’s any movement from the enemy, they bomb immediately,” he said.

The new firebase is part of a creeping U.S. buildup in Iraq since troops first returned to the country with a contingent of 275 advisers, described at the time by the Pentagon as a temporary measure to help get “eyes on the ground.”

Now, nearly two years later, the official troop count has mushroomed to 4,087, not including those on temporary rotations, a number that has not been disclosed.

The troops are moving outside the confines of more established bases to give closer support to the Iraqi army as it prepares for an assault on the northern city of Mosul — putting them closer to danger.

The shift to give closer support to Iraqis comes at a time of political turmoil in Baghdad, which is threatening the legitimacy of Iraqi Prime Minister Haider al-Abadi, the key partner for the United States. Iraqi commanders said they are concerned that the crisis will complicate and slow progress on the battlefield.

It was inside Firebase Bell, a few miles outside Makhmour, a small mixed Arab and Kurdish town on the edge of Iraq’s northern Kurdish region, that Marine Staff Sgt. Louis Cardin was killed on March 19 in a rocket attack, days after the Marines arrived here.

The area is prone to attack. Islamic State fighters sneak out at night to position explosives on the roads here and have sent a steady flow of suicide bombers to Iraqi army and Kurdish positions.

Before the U.S. troops and their M777 Howitzers moved here, the base came under regular fire.

A propaganda video released recently by the Islamic State showed a montage of clips of rockets and mortar rounds being launched toward the Iraqi army positions around Makhmour.