Syrian activists say Daulah Islamiyah militant group, formerly known as ISIS, have seized three towns in the eastern city of Deir Ezzor.
Syria Monitoring Organization for Human Rights said they were worried about the fall of the three cities will be followed by a massacre of indigenous people who live in those cities.
Last week dozens of people from the tribal fighters were killed after ejecting Daulah Islamiyah.
Syrian government troops continued to launch attacks, some of them with air seragan, to groups with ties to al-Qaida.
In another development, Iraqi Kurdish officials said at least 20,000 people, were trapped in the mountains by the fighters Daulah Islamiyah, escaped to a safer area in Kurdistan.
Officials said these people get away with the help of Kurdish soldiers.
Thousands of other civilians, most of the Yazidi people, still ensconced in the mountains in the middle of the scorching summer weather, while supplies of food and water is not sufficient.
They get food from aid being dropped by British planes and the United States. BBC