WASHINGTON (Reuters) - A fledgling U.S. base in northern Iraq came under attack again on Monday from Islamic State and even drew a threat from an Iran-backed Shi’ite militia, two days after a U.S. Marine there was killed in a rocket attack.

Firebase Bell, as the artillery outpost is called, is the first independent U.S. base of its kind in Iraq since the return of American forces to the country in 2014 and is the latest sign of deepening U.S. military involvement in the conflict.

Bell’s existence was meant to be kept secret until it was deemed operational, the U.S. military said, but Islamic State appeared to learn of the U.S. presence before the American public did.

The Sunni militant group launched a March 19 attack with Katyusha rockets, killing Marine Staff Sergeant Louis Cardin and injuring others in Cardin’s company-sized detachment of less than 200 troops.

Colonel Steve Warren, a Baghdad-based spokesman for the U.S.-led coalition, disclosed another attack on Monday, this time by a squad of Islamic State fighters who got close enough to the base to stage a failed attack with small arms.

Warren acknowledged adjustments were being made to strengthen the defenses.

“We are continuing to improve our fighting position, so to speak, to ensure that we’ve got the best ability to protect ourselves,” Warren told a news briefing.

The base was also drawing attention from an Iranian-backed Shi’ite militia, which said it would treat U.S. Marines deployed to the base as “forces of occupation” and would deal with them.

President Barack Obama has pledged to avoid a large-scale U.S. ground deployment in Iraq and to focus on enabling local forces. But the U.S. military has become increasingly involved on the ground, sending in U.S. special operations forces and, most recently, a detachment from the 26th Marine Expeditionary Unit.

Warren said the U.S. Marines took position at Bell a couple of weeks ago, aiming to help protect U.S. advisors now working with thousands of Iraqi forces at an adjoining base. The position was still behind the front lines.

Warren declined to say whether the Marines might also be able to provide offensive support when Iraqi forces advance toward the city of Mosul, Islamic State’s power center in Iraq.

“I’m not going to tell you you’re wrong, but I’m also not going to telegraph our punches,” Warren said.