The newly established American firebase in northern Iraq came under attack again Monday just two days after a rocket attack killed a U.S. Marine staff sergeant, a defense official said.

The company of Marines at the new outpost known as "Firebase Bell" reported small-arms fire Monday morning from a "squad size" team of  Islamic State fighters who likely "infiltrated" the area around the Iraqi and U.S. military facilities, said Army Col. Steve Warren, a Defense Department spokesman in Baghdad.

No American injuries were reported, Warren said.

The existence of the firebase, which was established last week, was reported publicly for the first time after Marine Staff Sgt. Louis Cardin was killed by a Katyusha rocket Saturday, the first attack on the site.

Cardin deployed to Iraq with a company of between 100 and 200 Marines from the 26th Marine Expeditionary Unit to help set up the new firebase near the expanding Iraqi military base at Makhmour, a key staging area for the Iraqi Army's planned assault on Mosul.

Makhmur is the location for the Iraqi's Nineveh Operations Center, a primary base from which the U.S. can support Iraqi forces when they eventually mount at attack on Mosul, which fell to the Islamic State in June 2014.

Iraqi military leaders have amassed several thousand troops in the area for the upcoming operation.  It's unclear when a full-scale invasion of Mosul might begin.

The Marines are considered "temporary" and are not counted as part of 3,800 U.S. troops that are currently authorized for full-time deployment to Iraq, Warren said. He declined to provide an total number of American boots on the ground in Iraq.

The Daily Beast reported Monday that the total number of U.S. military forces in Iraq, including the "temporary" troops, is more than 5,000.

The new firebase is about 15 to 20 kilometers from the "forward line of troops," or FLOT, that separates the Kurdish and Iraqi controlled zone of northeastern Iraq from the Islamic State-held territory in the Tigris River valley.

The Marines at Firebase Bell are operating field artillery to boost force protection at the nearby base in Makhmour. There are no Iraqi forces at Firebase Bell, Warren said.

"This is the first time we have established a spot that is only Americans," Warren said. He said the firebase is a separate enclosed facility that is very close to the headquarters installation at Makhmour.

Warren compared the U.S. advisers at Makhmour to the Americans who were deployed to Anbar province's al-Assad Air Base and Taqaddum Air Base providing support for Iraqis during the attack on Ramadi last year.

The Americans at Makhmour will be providing similar combat advice and support for Iraqi units planning an attack on Mosul, the largest city under Islamic State control.

The Makhmour area has been the scene of intense fighting between Kurdish Peshmerga troops and Islamic State militants, also known as ISIS or ISIL.  ISIS has launched mustard gas attacks in that area. Kurdish fighters also stopped an attempted attack there in November when the terror group tried to use mortar rounds filled with chlorine gas.

Eight other Marines were injured in the rocket attack Saturday that killed Cardin; several were evacuated to Landstuhl Regional Medical Center, an Army facility in Germany.

Cardin, a 27-year-old field artilleryman, was assigned to Battalion Landing Team, 2nd Battalion, 6th Marines.

Cardin, of Temecula, California, joined the Marine Corps in June 2006 and was based at Camp Lejeune, North Carolina. He deployed to Iraq once before and to Afghanistan three times.