Marines from an amphibious task force have left their ships in the Middle East and deployed to Syria, establishing an outpost from which they can fire artillery in support of the fight to oust the Islamic State from the northern Syrian city of Raqqa, defense officials said.

The deployment marks a new escalation in the U.S. war in Syria, and puts more conventional U.S. troops in the battle. Several hundred Special Operations troops have advised local forces there for months, but the Pentagon has mostly shied away from using conventional forces in Syria. The new mission comes as the Trump administration weighs a plan to help Syrian rebels take back Raqqa, the de facto capital of the Islamic State. The plan also includes more Special Operations troops and attack helicopters.

The force is part of the 11th Marine Expeditionary Unit, which left San Diego on Navy ships in October. The Marines on the ground include part of an artillery battery that can fire powerful 155-millimeter shells from M-777 Howitzers, two officials said, speaking on the condition of anonymity because of the sensitivity of the deployment.

The expeditionary unit’s ground force, Battalion Landing Team 1st Battalion, 4th Marines, will man the guns and deliver fire support for U.S.-backed local forces who are preparing an assault on the city. Additional infantrymen from the unit will provide security, while resupplies will be handled by part of the expeditionary force’s combat logistics element. For this deployment, the Marines were flown from Djibouti to Kuwait and then into Syria, said another defense official with direct knowledge of the operation.

The official added that the Marines’ movement into Syria was not the byproduct of President Trump’s request for a new plan to take on the Islamic State but that it had “been in the works for some time.”

“The Marines answer a problem that the [operation] has faced,” the official said. He added that they now provide “all-weather fires considering how the weather is this time of year in northern Syria.”

Lt. Gen. Stephen Townsend, the top U.S. general overseeing the campaign against the Islamic State, has previously said that a small number of conventional soldiers have supported Special Operations troops on the ground in Syria, including through a truck-mounted system known as the High Mobility Artillery Rocket System, or HIMARS. The defense official with knowledge of the deployment said Wednesday that the Marines and their howitzers will supplement, rather than replace, those Army units.

The new Marine mission was disclosed after members of the Army’s elite 75th Ranger Regiment appeared in the Syrian city of Manbij over the weekend in Strykers — heavily armed, eight-wheel armored vehicles. Defense officials said they are there to discourage Syrian or Turkish troops from making any moves that could shift the focus away from an assault on Islamic State militants.

The Marine mission has similarities to an operation the Marine Corps undertook about a year ago when the U.S. military was preparing to support an assault on the Iraqi city of Mosul. In that case, a force from the 26th Marine Expeditionary Unit, from Camp Lejeune, N.C., established a fire base south of the city in support of Iraqi and Kurdish troops who were then carrying out operations to isolate Mosul from Islamic State-held territory around it.

The existence of the outpost near Mosul, originally named Fire Base Bell, became public after it was attacked by rockets March 19, 2016, killing Staff Sgt. Louis F. Cardin and wounding at least four other Marines. Defense officials said at the time that they had not disclosed the deployment of Marines there because the base was not fully operational, although photographs released by the Defense Department shortly afterward showed Marines launching artillery rounds a day before Cardin’s death.

For the base in Syria to be useful, it must be within about 20 miles of the operations that U.S.-backed forces are carrying out. That is the estimated maximum range on many rounds fired from the M-777 howitzer. GPS-guided Excalibur rounds, which the Marines also used after establishing Fire Base Bell, can travel closer to 30 miles. Fire support for the Mosul operation has since been turned over to the Army.