01:56 - Source: CNN
U.S. Marine killed in ISIS attack in Iraq

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At least two ISIS fighters were killed in the clash, no U.S. forces were killed or wounded

The U.S. will not confirm the total number of U.S. troops in Iraq

Washington CNN —  

For the second time in three days, the first U.S. military firebase in Iraq has come under attack from ISIS. The base, known as Firebase Bell, came under small arms fire from a group of about 10 ISIS fighters who also attacked a nearby U.S./Iraqi installation at Makhmur in northern Iraq.

No U.S. personnel were wounded, and at least two ISIS fighters were killed, according to Army Col. Steve Warren, the coalition spokesman. Warren said he didn’t have an exact distance of how close the gunmen came to the U.S. security perimeter, saying only “a couple of hundred meters, I would assume.”

The remaining attackers “ran away in fear,” Warren said.

A U.S. Marine stationed at Firebase Bell was killed by an ISIS Katyusha rocket attack on Saturday. Eight U.S. troops were also wounded in the attack. Three were medivaced to Germany where one is described as having serious injuries a defense official told CNN.

The U.S. believes the firebase was deliberately targeted by ISIS fighters who were aware it was there, Warren said. He noted the helicopters bringing in troops and equipment were readily visible. But the defense official noted that there are several ways ISIS could have learned the Marines were there, “You can’t hide 200 Marines,” the official said.

The U.S. currently has between 4,500 and 5,000 troops in Iraq on a regular basis, about 1,000 over the stated limit. This includes the 200 Marines that are at the firebase along with 200 special operations forces whose presence is not publicly acknowledged the official said.

The coalition refused to publicly reveal the exact number of U.S. troops currently in Iraq, with Warren only telling reporters that it was within 3,870 troops. “There’s been a decision made not to release that number,” he said.

Warren added that Congress had been informed of the methodology the military used to calculate troop levels.

On Sunday, the coalition announced that in consultation with the government of Iraq, the U.S. “has assigned a detachment of U.S. Marines from the 26th Marine Expeditionary Unit to the support of Iraqi Security Force and Coalition ground operations. The detachment from the 26th MEU will add to the Marines and Sailors currently in Iraq” supporting the effort against ISIS.”

The U.S has similar artillery at two other large Iraqi bases, but Bell is the only known firebase. That terminology signifies this is a small remote location designed to use its artillery to support infantry troops at forward locations.

The Marines will be using their field artillery in the coming weeks to help defend Iraqi troops as they move towards Mosul. For now, the artillery is to defend another nearby base where American troops are getting 5,000 Iraqi forces ready for the operation to retake Mosul.

CNN’s Ryan Browne contributed to this report