The Marines who set up the first stand-alone U.S. firebase in the campaign to defeat ISIS came under attack again Monday following the combat death Saturday of Staff Sgt. Louis F. Cardin from rocket fire in northern Iraq.

"There was some small arms fire this morning" aimed at the new Firebase Bell and troops of the Iraqi Army's 15th Division near the town of Makhmour, which has become a staging base for the projected assault to retake the ISIS stronghold of Mosul, said Army Col. Steve Warren, a spokesman for Combined Joint Task-Force-Operation Inherent Resolve.

The small arms fire from a squad-sized unit of ISIS fighters was "ineffective" and no casualties resulted. Two ISIS fighters were killed "and the rest ran away," Warren said, but their ability to infiltrate Iraqi lines to get close to the Marine firebase, coupled with the rocket fire Saturday that killed Cardin, suggested that they had zeroed in on the new outpost.

In a video briefing from Baghdad to the Pentagon, Warren said that the presence of Firebase Bell on the frontlines did not represent an "escalation" of the U.S. war effort, which has been limited to a train, advise and assist role on the ground, but he would not rule out that the Marines would support the Iraqis when they move towards Mosul.

The Marines' presence was temporary and "their mission now is to provide force protection. When Iraqi forces begin to move, then we'll talk about what our forces are doing associated with that," Warren said.

Warren said a company-sized force of about 200 Marines from the 26th Marine Expeditionary Unit began moving ashore from the Kearsarge Amphibious Ready Group in the Persian Gulf about two weeks ago into the area near Makhmour, about 60 miles southeast of Mosul.

The amphibious assault ship USS Kearsarge (LHD-3) was supported by the amphibious transport dock USS Arlington (LPD-24) and the dock landing ship USS Oak Hill (LSD-51).

The Marines brought with them four 155mm howitzers artillery pieces to provide force protection mainly to the approximately 100 Special Forces advisers who have been in the area for several weeks to help prepare Iraq's 15th Division to move on Mosul.

"They're staging, preparing, getting ready for battle," Warren said of the Iraqis, "so we have advisers there" to help "synchronize air and ground operations when that time comes."

"This is the first time we have established a spot that is only (for) Americans" in the campaign against ISIS in Iraq, Warren said. The U.S. has set up similar firebases at Taqqadam and at the al-Asad airbase in southern Anbar province, but Firebase Bell was the first to stand alone with its own perimeter.

The firebases at Taqqadam and al-Asad were within the confines of large Iraqi bases while Firebase Bell was set up a few hundred meters from Iraqi troops, Warren said. The mission of the Marines at Firebase Bell was "no different from what we're doing in Taqqadam and al-Asad -- they're able to fire back very quickly when the enemy shoots," he said.

"They've got some very powerful cannons with them," Warren said of the 155mm howitzers. "They will fire defensively," but "if they seem something that's a threat to them, they'll make a decision." The Marines also do not have to get permission from the Iraqis to shoot, Warren said.

ISIS fighters fired two rockets, believed to be Katyushas, at Firebase Bell on Saturday, Warren said. One landed without effect but the second landed inside the firebase, killing Staff Sgt. Cardin, he said. The Marines launched counter-battery fire but the initial assessments were that the ISIS fighters had already moved out of the area, Warren said.

Cardin, a 27-year-old field artilleryman, was assigned to the 2nd Battalion, 6th Marine Regiment, 26th MEU based at Camp Lejeune in North Carolina. He had deployed once before to Iraq and three times to Afghanistan.

Gen. Robert B. Neller, the Marine commandant, said on Twitter there were "heavy hearts" across the Marine Corps. "God bless the family of SSgt Louis F. Cardin -- killed in Iraq 19 Mar. Semper Fidelis," Neller said.

Army Gen. Lloyd Austin, commander of U.S. Central Command, also paid tribute to Cardin, saying his "service and his many important contributions will long be remembered by his fellow Marines, his teammates at United States Central Command, and a grateful nation."

He added, "Our gratitude and heartfelt condolences go out to this young man's family and friends."

The deployment of the Marines to Makhmour did not increase the number of U.S. troops in Iraq beyond the 3,870 that have been agreed to by the Iraqi government, Warren said. Warren would not give the exact number of U.S. troops now in Iraq but said "we're within the cap."

The Pentagon and the Marine Corps had kept the deployment of the Marines to northern Iraq secret until the attack that killed Staff Sgt. Cardin. Warren said information on the deployment had been held back by commanders in Iraq for security reasons to allow the firebase to become established before its presence was announced.

-- Richard Sisk can be reached at Richard.Sisk@Military.com.