Tribesmen in three villages in eastern Syria have driven out Islamic State (IS) militants, in a rare display of local resistance to the group.
Four days of fighting left nine IS fighters, three tribesmen and five civilians dead, UK-based Syrian opposition activists say.
The jihadists' actions in the Ashara area had bred resentment locally, another activist based in Turkey said.
In neighbouring Iraq, IS fighters fought Kurdish forces at Zumar.
IS is dedicated to building an Islamist state in Syria and Iraq.
It built on its gains in Syria this summer to sweep through western and northern Iraq with support from local Sunni Muslims, overrunning the city of Mosul and threatening the capital Baghdad.
In recent weeks, it also expanded territory under its control in Syria, capturing parts of the oil-rich province of Deir Ezzor.
Formerly known as the Islamic State in Iraq and the Levant (Isis), it has been accused of atrocities in areas under its control, carrying out mass executions of Shia Muslim prisoners and forcing out other non-Muslims such as Mosul's ancient Christian community.
Fighting erupted on Wednesday after IS detained three tribesmen, the UK-based Syrian Observatory for Human Rights and Turkish-based activist Mustafa Osso said.
The detentions apparently broke an agreement with local people in the villages of Kishkiyeh, Abu Hamam and Granij, and tribesmen torched the IS headquarters in Ashara in retaliation, the Observatory said.
IS reportedly rushed in reinforcements from the Iraqi border town of Qaim but were forced out of the villages.
Tribesmen also captured the nearby Tanak oil field, according to the Observatory.
"There has been wide resentment recently because of Islamic State's acts," Mr Osso told AP news agency.
"This is a very important area for Islamic State because it is rich with oil and borders Iraq."
IS is one of the larger groups fighting Syrian President Bashar al-Assad.
More than 150,000 people have been killed in Syria and more than nine million have been forced to leave their homes since the uprising against Mr Assad began in March 2011.
In Iraq, jihadists attacked Kurdish forces in Zumar on Friday and fighting has continued since, with at least 14 Kurds and an unknown number of IS fighters killed.
There were conflicting reports on whether Kurdish peshmerga fighter or IS forces held the town.
"Many Islamic State vehicles are wandering the town of Zumar and I can also see the flags on top of buildings," on resident told Reuters news agency.
Control of Zumar would give the jihadists access to a small oilfield and nearby refinery, adding to four oil fields they have already seized, according to Reuters.
Kurdish forces have become the only effective opposition to IS in northern Iraq since government troops were driven out of Mosul and other areas in June.
Reports say Islamic State fighters were also involved in deadly clashes overnight with Iraqi troops in the mainly Sunni town of Jurf al-Sakhar, 50km south-west of Baghdad.
At least nine soldiers are said to have been killed.