Photographs and information released by the U.S. military provide new details about a fire base recently established by the Marine Corps in northern Iraq to target Islamic State militants, including its austere nature and how troops arrived there last week.

The base’s existence was kept secret by the Pentagon until Monday, two days after Marine Staff Sgt. Louis Cardin was killed there by an incoming rocket. He was a member of the 26th Marine Expeditionary Unit, an amphibious task force from Camp Lejeune, N.C., that typically travels on Navy ships and has been called on to temporarily bolster the campaign against the Islamic State. The base includes four 155mm M777A2 Howitzer cannons, and is located a few hundred yards from a larger Iraqi base outside the town of Makhmour where U.S. military advisers have been training Iraqi troops.

Information released with the photos details how the outpost was established by Marines who flew in on Army CH-47 helicopters from Camp Taji, a base on the outskirts of Baghdad. They arrived under cover of darkness March 17, and were launching artillery at Islamic State infiltration routes by the following day, the military said. The M777A2 can launch rounds at least 22 miles — well into nearby Islamic State territory, but not as far as the Islamic State-controlled city of Mosul, which is about 60 miles northwest.

The Marines are labeled in captions released with the photos as members of Task Force Spartan, a designation that was not previously disclosed. About a company of Marines — typically a couple of hundred troops — are involved, Defense Department officials said. Other Marines provide force protection at the larger, nearby Iraqi base where the U.S. advises the Iraqis.

One of the photos shows a round getting launched from the fire base’s artillery pit. Thick grass surrounds it, with a dirt berm several dozen yards away providing some cover from outside attacks. A few “T-walls” also are lined up near the berms, giving the Marines some additional protection. The berms don’t yet incorporate HESCO, wire-and-cloth containers that are filled with dirt and are standard on U.S. battlefield bases to protect troops from outside attacks.

The photos were released late Wednesday as the Iraqi government announced that its battle to take back Mosul from the Islamic State has begun. U.S. officials have struck a more measured tone, saying shaping operations in advance of any assault are still ongoing some distance from the city.

It isn’t clear how long the Marines will remain at Fire Base Bell. The “force cap” set by the Obama administration allows up to 3,870 service members to be deployed to Iraq, but the Marines and some military advisers don’t count against that figure. The actual number of U.S. troops in Iraq exceeds 5,000, Defense Department officials said this week. That’s a sensitive issue for the Obama administration and Prime Minister Haider al-Abadi, both of whom have expressed a desire to not expand U.S. operations in Iraq any more than required.

If the Marines need to be removed from the battlefield in order to satisfy any agreements reached by the United States and Iraq, it’s possible they could be replaced by another unit of U.S. forces on a deployment considered temporary. It’s also possible that Fire Base Bell could be taken down in favor of establishing another one closer to Mosul as Iraqi security forces push toward the city.

Here are other photos released of Fire Base Bell: