BEIRUT — The extremist Islamic State group shot and beheaded hundreds of tribesmen from eastern Syria over the past two weeks after crushing an uprising they led amid the country's civil war, activists said Monday.

The killing of members of the Shueitat tribe come as Islamic State group fighters close in on the last government-held army base in the region. Syrian warplanes bombed the extremists' positions Monday in an attempt to halt their advance.

A Turkey-based activist who is originally from Deir el-Zour and is in contact with people in the province told The Associated Press that as many as 200 members of the Shueitat tribe have been killed by Islamic State members. On Saturday, the Britain-based Syrian Observatory for Human Rights put the death toll among the tribesmen at 700.

The activist spoke on condition of anonymity for fear that family members in Syria might be harmed. Differences in casualty figures are common in the aftermath of attacks in Syria.

Last week, Islamic State fighters crushed the tribal uprising against their rule in eastern Syria after three days of clashes near the border with Iraq. Tribesmen expelled the jihadi fighters from the villages of Kishkiyeh, Abu Hamam and Granij earlier this month before the Islamic State launched a counteroffensive that killed dozens, activists said.

"They considered all members of the Shueitat tribe apostates because they rose against them," the activist said. "Some men were taken out in the fields and beheaded them while others were shot in the head."

The Islamic State group has declared a self-styled caliphate in territory it controls along the Iraqi-Syrian border, imposing a harsh interpretation of Islamic law. They have executed hundreds of people in Iraq and Syria over the past two months, including some who were beheaded and their heads were later displayed in a public square in the northern Syrian city of Raqqa.

Last month, jihadis overran the sprawling Division 17 military base in Raqqa province, killing at least 85 soldiers. Two weeks later, Islamic State fighters seized the nearby Brigade 93 base after days of heavy fighting.

They now are closing in on the Tabqa air base, the last position held by Syrian government troops in Raqqa province.

The Observatory reported intense clashes Monday between troops and Islamic State fighters on the edge of the villages of Ajil and Khazna near the Tabqa air base. It said there were casualties on both sides.

The Raqqa Media Center, an activist collective, said Islamic State group fighters captured four villages near the air base, including Ajil.

The Observatory and RMC also reported intense air raids for the second day on Raqqa's provincial capital.