The ISIS captured Bukamal town in Syria's Deir al-Zour province after an "intense" battle with the Al Qaeda-linked Nusra Front, Xinhua reported citing the London-based watchdog Syrian Observatory for Human Rights.
The ISIS has started advancing towards al-Shahel town, a stronghold of the Nusra Front, in the eastern Syrian countryside of Deir al-Zour, the observatory said.
The Nusra Front and the ISIS have been fighting since early this year.
Their battles have claimed the lives of more than 7,000 insurgents and civilians since the beginning of 2014 in northern and eastern Syria, according to the Observatory.
The two rebel groups have been fighting over Bukamal due to its strategic location on the border with Iraq and its oilfields.
The ISIS is advancing towards al-Shahel town, a stronghold of the Nusra Front
Meanwhile, the UN Assistance Mission for Iraq (UNAMI) said as many as 2,417 Iraqis, mostly civilians, were killed in terrorist attacks and violence in June.
The UNAMI statement said that 1,531 civilians, including civilian police personnel, and 886 members of the security forces were killed, while 1,763 civilians were wounded, in addition to 524 security members in terrorist and violent acts last month.
In another development, the ISIS staged a military parade in Syria's northern city of al-Raqqa, displaying weaponry and military gear it looted from military depots in neighbouring Iraq, activists said Tuesday.
Meanwhile, incidents of bombing and firing were reported in the vicinity of the building in Iraq's Tikrit town which is housing 46 Indian nurses.
They have taken refuge in the basement, the Indian government said Tuesday, expressing hope that they will be evacuated safely.
In New Delhi, External Affairs ministry spokesperson Syed Akbaruddin said it was a "delicate situation" and Iraqi authorities had been informed about the location of nurses.
He said that despite the difficulties, the government hoped to extricate the nurses safely.
"The nurses remain safe, unharmed... Nurses are in touch with the (Indian) mission. The mission is advising them," he said.
Referring to Indians in non-conflict areas in Iraq, he said that mobile teams were active and 233 Indians had been booked to fly out Tuesday.
He said around 1,000 of the Indian nationals who had been contacted had expressed their desire to return but almost a similar number were keen to stay back.
Asked about 39 Indians in captivity in Mosul, Akbaruddin said that they were unharmed.
On June 10, Sunni militant groups, including those linked to the Islamic State in Iraq and Greater Syria (ISIS), an Al Qaeda offshoot, took over the key cities of Mosul and Tikrit, as well as other northern and western parts of the Sunni heartland as Iraqi security forces were driven into disarray when waves of surprise attacks were mounted against them.