The US-led coalition air campaign against ISIS militants in Iraq and Syria has dragged on for months, but the US airmen mounting the raids haven't lost track of time.

During Christmas Day airstrikes against the terrorist group, at least one US pilot donned a Santa hat, photos of which were shared by the US Air Force, as first spotted by international monitoring group Airwars.

A US F-16 pilot over Iraq wears a Santa hat during strikes against ISIS on Christmas Day, December 25, 2016. US Defense Department

Coalition air forces mounted 18 strikes against ISIS targets in Iraq and Syria on December 25.

Near Raqqa, the group's de facto capital city in Syria, 11 strikes were directed at ISIS tactical units, fighting positions, vehicles, and weapons, including ISIS' apparent go-to of late: a vehicle-borne improvised explosive device.

Near Mosul, the second-largest city in Iraq and the terror group's last stronghold in that country, two strikes were directed at ISIS tactical units, fighting positions, vehicles, weapons, and infrastructure.

According to a release from Operation Inherent Resolve, all aircraft involved in Christmas Day strikes returned to base safely.

The US-led coalition has mounted hundreds of strikes against ISIS in Iraq and Syria, targeting the terror group's vehicles, weapons, buildings, fighting positions, resources, and tunnel networks.

An F-16 pilot over Iraq can been seen wearing a Santa hat during a Christmas Day operation, December 25, 2016. US Defense Department

Progress against ISIS in Syria appears to have been set back by the Syrian and Russian governments' concentration on Aleppo, which fell earlier this month after years of resistance.

Moscow and Damascus' focus on the city reportedly allowed the terrorist group to make progress elsewhere in the country, while a campaign against Raqqa in the east is still in its early stages.

The six-week-old offensive against Mosul, led by Iraqi government forces, appears to have been paused, holding in what one US official called an "operational refit" period earlier this month.

Progress against ISIS in Mosul has been slow, as the confines of the city and the immense number of civilians still in it make military advances hard to come by.

Iraq forces have only retaken about one-quarter of the city since launching their offensive in mid-October.

A US Army soldier takes position at the US section of a base for Iraqi army and Kurdish peshmerga forces in Makhmour, southeast of Mosul, Iraq, December 23, 2016. REUTERS/Khalid al Mousily

US forces on the ground in Iraq appear to be stepping up their involvement in the fight for Mosul.

As a recent Reuters report quotes the commander of the main US unit on the ground in Iraq:

" We have always had opportunities to work side-by-side, but we have never been embedded to this degree ... That was always a smaller niche mission. Well, this is our mission now and it is big and we are embedded inside their formations."

Many civilians have found themselves in the cross-fire, especially in Iraq, where they have been caught between ISIS and Iraqi fighters on the ground as well as in the path of ongoing airstrikes.

In the 10 days before Christmas, Airwars documented reports — some of them contested — indicating that more than 50 civilians were killed by air and ground fire from coalition and Iraqi forces.

ISIS continues to menace civilians as well.

The terrorist group has fired on civilian areas of Mosul, including liberated sections of the city, and a Human Rights Watch report issued on Tuesday states that the terror group executed at least 13 people — including two boys — in villages south of Mosul where locals mounted an effort to expel the group's fighters.