Islamic militants have crushed a tribal uprising against their rule in eastern Syria after three days of clashes in three eastern villages near the border with Iraq, activists said Monday, Aug 11, according to the Associated Press.

The armed revolt by the Shueitat tribe in the eastern province of Deir el-Zour was the first sign of local resistance to the Islamic State group since its fighters swept into the province.

The Britain-based Syrian Observatory for Human Rights and Turkey-based activist Thaer ak-Deiri said that Islamic State group fighters regained control of three villages of the Shueitat tribe on Sunday after being expelled earlier this month.

The Observatory said Islamic State fighters beheaded two tribesmen after they fled to the nearby village of Shaafa. It had no immediate word on other casualties in the area.

Clashes over the past two weeks left more than a dozen people dead and both sides.

The extremist group has taken over much of northern and eastern Syria as well as western and northern Iraq. The group has declared a self-styled caliphate in territory it controls along the Iraqi-Syrian border, imposing a harsh interpretation of Islamic law.

The clashes in eastern Syria came as Islamic State fighter tightened its siege on a major military air base in the town of Tabqa in the northern province of Raqqa. The air base is the last army position in the Raqqa province that is an Islamic State stronghold.

The Observatory’s chief Rami Abdurrahman said Islamic State fighters are bombarding the base with artillery as they appear to be preparing to storm it.

Last week, Islamic State fighters seized the nearby Brigade 93 base after days of heavy fighting and late last month captured another base in which they took dozens of prisoners whom some of them were later beheaded and their bodies paraded in one of Raqqa’s main squares.

Syria’s conflict began in March 2011 as a popular uprising against President Bashar Assad’s rule, but turned into an insurgency after government forces violently cracked down on demonstrators. It has since become civil war with sectarian overtones. Over 170,000 people have been killed in Syria in more than three years of fighting, activists say.