AMMAN (Reuters) - Syrian rebels battled army units on Thursday for control of districts of Damascus on the second day of a rebel offensive which aims to shake President Bashar al-Assad's hold on the capital, a rebel captain and an opposition activist said.

Elite Republican Guard units based on the imposing Qasioun Mountain in the middle of the city fired artillery rounds and rocket launchers at the eastern neighborhood of Jobar and at the southern ring road, where rebels have overrun roadblocks and army positions, the sources said.

They put the overnight death toll at 30 people, mostly from heavy army bombardment on the rebellious neighborhoods of Jobar, Zamalka and Hajar al-Aswad.

Jobar and Zamalka are situated near security compounds housing forces from Assad's Alawite sect, an offshoot of Shi'ite Islam that has dominated power in Syria since the 1960s.

Hajar al-Aswad is close to the southern entrance of the capital and the main highway to the city of Deraa and the Jordanian border.

"Jobar is the most contested district and the regime is bombarding it heavily," said Captain Islam Alloush of the Liwa al-Islam rebel unit. He said the army was massing forces to take back a major junction on the ring road.

Syrian authorities have banned most independent media from the country, making it difficult to verify events on the ground.

Alloush said the aim of the rebel offensive was not to take central Damascus. That would not be attempted while Assad's forces controlled major bases to the rear of the rebel forces, in Muleiha district and in the town of Adra, he said.

"The objective is to take out the sniper positions and fortifications that form part of the regime's defense line on Damascus, not to advance too quickly without having the proper support," he said.

The opposition activist in Damascus said the offensive was being led by Sunni officers who had defected from the army, and aimed to cut Assad's command and control lines from the centre of the city to its outskirts.

The rebels are using anti-aircraft guns, mortar rounds and armored vehicles captured from Assad's forces over the past few months, according to opposition sources.

(Reporting by Khaled Yacoub Oweis, Amman newsroom; Editing by Pravin Char)