A Kurdish People's Protection Units fighter walks beside dead bodies of Islamic State fighters after the Kurds regained control of Al-Qassaib village, activists said, in Qamishli, northeastern Syria, Dec. 25, 2014. Reuters

Islamic militants in Syria have executed a man they said belonged to a group that had attacked their fighters and detonated their vehicles in al-Mayadin, near the border with Iraq, amid a rise in ambushes against Islamic State forces by Syrian groups, according to Reuters. The man was reportedly beheaded and his body crucified in a public park.

Four other men were similarly executed in Deir al-Zor in eastern Syria, which is almost completely controlled by the militant group also known as ISIS. They were accused of having ties with the Syrian authorities, according to the Syrian Observatory for Human Rights, which has been monitoring the war for almost four years. "One was arrested for smoking a cigarette,” the head of the monitoring group said. “Only later did Islamic State accuse him of being an informer for the regime.”

ISIS controls most of Deir al-Zor province, a strategic stronghold that links the Islamic group to its followers in Iraq. Guerrilla-style Syrian groups have formed in the region in response to ISIS’ mounting violence. One such group has reportedly killed more than 100 ISIS fighters, Reuters reported.

Three months of U.S. airstrikes in Syria have done little to keep ISIS from expanding its territory there, according to the Wall Street Journal. The U.S. began airstrikes against ISIS in September. The failure of that campaign in Syria has cast doubt on President Barack Obama’s strategy in the region, although airstrikes against Islamic militants in Iraq have had some success, military experts have said.

“Certainly ISIS has been able to expand in Syria, but that’s not our main objective,” one U.S. senior defense official told the Journal. “I wouldn’t call Syria a safe haven for ISIL, but it is a place where it’s easier for them to organize, plan and seek shelter than it is in Iraq.”

ISIS, also known as ISIL and Daesh, has been gaining ground in Syria and Iraq, where the group has been mounting an aggressive campaign since June.