Unidentified gunmen attacked Islamic State militants in the eastern Syrian town of al-Mayadin overnight, killing more than 12.
The attacks in Deir al-Zor province were carried out by two separate groups of gunmen, the Syrian Observatory for Human Rights reported. Islamic State, which seized nearly all of Deir al-Zor province last year, has faced a wave of hit-and-run attacks by unidentified gunmen in the area.
There was no immediate claim of responsibility for the attacks. Syrian state media attributed similar attacks in January to "popular resistance" to Islamic State, which has brutally suppressed any opposition to its control.
The first group of assailants opened fire on an Islamic State patrol, killing 12 gunmen, before attacking Islamic State members who were guarding a courthouse. They were riding at least two motor bikes.
A second group of gunmen meanwhile attacked an Islamic State checkpoint in the same town, killing and injuring an unknown number of the Islamic State militants, the Observatory reported.
Al-Mayadin is located about 60 km (40 miles) from the border with Iraq.
While it has lost control of most of Deir al-Zor province to Islamic State, the Syrian government still controls part of the Deir al-Zor city.
A U.S.-led alliance seeking to roll back the group in both Syria and Iraq has dismissed the idea of partnering with President Bashar al-Assad in the fight against Islamic State, seeing him as part of the problem.
The Syrian military said on Saturday it had killed a leading member of Islamic State in an air strike in Hama province in central Syria. The Observatory said two commanders and 26 Islamic State militants had been killed in that attack.
The Syrian military and allied militia including the Iranian-backed Hezbollah are fighting an array of insurgent groups, including both jihadists and mainstream rebels, across the country in the nearly four-year-old crisis.
The war is estimated to have killed around 200,000 people.
The Observatory said a rocket attack by government forces on Sunday killed 11 people and wounded at least 50 others in the town of Irbin in the rebel-held Ghouta area east of Damascus.