Syrian jets bombed on Saturday rebel-held eastern areas close to the border with Iraq under the control of the Islamic State in Iraq and the Levant, killing and injuring dozens in stepped up raids against the militant group since its Iraqi offshoot made stunning gains in northern Iraq.

Five raids killed at least 16 people and injured dozens more when bombs hit residential areas in the town of Muhassan just over 100 km from Iraq, a day after tribal elders in the town along the Euphrates River, pledged allegiance to ISIL.

The Syrian branch of the hardline Islamists, whose stated aim is to create a strict Islamic state straddling national borders, took over the town of Muhassan along with the Albulil and Albuomar, in the latest advance in eastern Syria adjoining territory the al-Qaeda splinter has seized in Iraq.

Syrian fighter jets were seen taking off from the rebel besieged Deir al-Zor military airport to bomb several areas under the control of ISIL and witnesses and activists said areas near the border with Iraq close to the city of Abu Kamal were bombed.

"(Syrian President Bashar al-) Assad's forces want to punish towns such as Muhassan for their support of ISIL. This bombing campaign is lending a hand to help Iraqi Shi'ite leader Maliki (Iraqi Prime Minister) to destroy ISIL after its victories," said Abdullah al-Mashour, an ISIL sympathizer from the town of Muhassan, by Skype.

The hardline Islamist group which has been involved in fighting rival insurgents for months to consolidate its grip on the oil rich Deir al-Zor province on the border with Iraq is staging an offensive to capture the remaining parts of the province it still does not control.

Opposition sources say their next targets are Shuheel, Mayadin and Abu Kamal, towns closer to the Syria border and now in the hands of rival Islamist groups, to allow ISIL to stretch territorially to Iraq, where family and tribal ties overlap.

ISIL already controls almost 70 per cent of the Deir al-Zor region, according to some rebel sources. Although some towns have been seized after deadly battles with rival groups, other tribal towns have been won over by ISIL without a fight through a mixture of coercion and copt leaders.

"Deir al-Zor is a prime prize for ISIL with its rich resource of oil and grains silos which it wants to further its goal," said Abu Thaher, a resident of Deir al-Zor contacted on Skype, referring to its mission of carving out an Islamic state.

"Their aim is to link these areas with Iraq's Anbar," he added.

ISIL's seizure of large amounts of weaponry and money from looted banks in Iraq after its capture of the city of Mosul have emboldened followers in Syria and instilled fear among tribal leaders in the eastern region, forcing many to make truces and accept their tutelage, some residents say.

"They are playing the tribal card and by winning tribal new allegiances by a carrot-and-stick approach, they are doing so without losing fighters," Aziz Abdul Rahman, a lawyer residing in Deir al-Zor, told Reuters by Skype.

Nevertheless ISIL has also sought to build strong support among the major Bakkir and al Akaidat tribes in Deir al-Zor, from where many of its rank and file are drawn.

The hardline group also has been trying to build strong tribal support in the city of Raqqa, where it runs its affairs and remains the only provincial capital in Syria under rebel control.

The group's headquarters were the target of intensive raids by al-Assad's air forces last week.

ISIL's toughest fighters said to be behind gruesome executions that earned the group a reputation for ruthlessness are mainly foreign jihadists, including Chechen Abu Omar al-Shishani, believed to have a crack force of over 800 fighters.-Reuters