Although U.S. President Barack Obama has pledged a number of times that there would be no American "boots on the ground" in the fight against the Islamic State (ISIS), the Pentagon appeared to have set this aside as it announced on Sunday that the U.S. is deploying more troops to Iraq.

The Pentagon announcement came after an ISIS rocket killed a Marine and seriously injuring others over the weekend, Fox News reported.

"It was a lucky strike by ISIS," a U.S. official said, adding that the attack took place in the northern Iraq town of Makhmur, about 75 miles southeast of the ISIS stronghold of Mosul.

The slain Marine—identified as Staff Sgt. Louis F.Cardin, of Temecula, California—was only the second American fatality in Iraq since combat operations began in August 2014, Fox News said.

The Pentagon declined to put a number on the additional troops to be deployed but said they will come from the 26th Marine Expeditionary Unit and will support Iraqi forces and international ground operations.

About 3,700 U.S. troops are now stationed in Iraq, leading air strikes against ISIS forces, supplying arms to the Kurdish peshmerga, carrying out humanitarian air drops and providing intelligence and support to the Kurdish and Iraqi armies, The Independent reported.

U.S. special forces have made occasional raids into ISIS territory. In October, they freed 70 hostages during a raid where a U.S. sergeant was killed.

The latest U.S. deployment is a significant step towards the use of conventional warfare tactics, according to the Independent. Such a shift in policy is a politically divisive move in the U.S. in the aftermath of the protracted and bloody Iraq war.

Speaking in January, U.S. Defense Secretary Ash Carter said: "We're looking for opportunities to do more, and there will be boots on the ground. I want to be clear about that."

It was not made clear whether Obama's earlier "no boots on the ground" pledges have already been set aside.

A top-ranking U.S. general in Iraq even expounded on Carter's statement, saying, "We have shifted from a pure counterinsurgency focus and are now preparing the [Iraqi security forces] to conduct what we refer to as combined arms operations.

"There is a good potential that we will need additional forces to provide those capabilities. The ability to integrate infantry, armor, artillery, air power, engineers and other assets on the battlefield provides the Iraqis with a decisive advantage over a static enemy dug in behind complex obstacle belts."

Meanwhile, airstrikes targeting ISIS fighters struck Mosul and surrounding areas over the weekend, sources told CNN.

CNN could not immediately independently verify the specifics of the claims, as the strikes hit inside ISIS-held territory.

The reports said Mosul University—which is considered a base for ISIS fighters—was hit.