Rebels and loyalists clashed on two key fronts near Damascus, as a missile hit Raqa city in northern Syria early Thursday, killing six people and wounding 30, a monitor said.

The frontline battles raged around the Qalamoun area that links Damascus to the central city of Homs and east of the capital where rebels are fighting to break a year-long siege, said the Syrian Observatory for Human Rights.

The early morning clashes were especially fierce around Qalamoun's Deir Attiyeh which rebels, including jihadist fighters, seized from the army last week, said the Britain-based group.

"The army is advancing around Deir Attiyeh," said Observatory director Rami Abdel Rahman, adding that loyalists including pro-regime militia were trying hard to take it back from opposition hands in order to secure the Damascus-Homs road.

In Eastern Ghouta, east of Damascus, battles raged in the Marj area days after rebels launched an offensive aimed at breaking the siege on opposition-held areas.

"Nine rebels were killed around Marj, as were three members of Hezbollah," Lebanon's powerful Shiite group which the Observatory says has sent hundreds of fighters to back the army.

In the northern city of Raqa, a surface-to-surface missile launched overnight from Damascus province killed at least six people and wounded at least 30 others, including two women, the monitor said.

Raqa is the only provincial capital in Syria to have fallen out of regime hands since the start of the country's war more than 32 months ago.

It is now under jihadist control, but activists have frequently accused the army of targeting only civilian areas of Raqa, rather than parts of the city where the feared Islamic State of Iraq and the Levant is positioned.

In Aleppo province's Atareb, ISIL executed Hassan Jazra and six members of his Ghuraba al-Sham battalion, after it had accused them of theft and looting.

In areas where it is powerful, ISIL has sought to establish itself as the sole power-broker, first by eliminating small rival groups over charges of corruption, then by opening fronts with bigger battalions.

Syria's war has killed more than 120,000 people and forced millions more to flee their homes.