* Residents say some of heaviest bombardment in conflict
* Bombardment focused on eastern Jobar district, resident say
* Rebels say seek to weaken Assad's lines of defence (Adds details, residents' quotes)
By Khaled Yacoub Oweis
AMMAN, Feb 7 (Reuters) - Syrian rebels battled army units for control of districts of Damascus for a second day on Thursday, part of a rebel offensive which aims to shake President Bashar al-Assad's hold on the capital, a rebel captain and opposition activists said.
Units of Assad's elite Republican Guard based on the imposing Qasioun Mountain overlooking the city fired artillery rounds and rockets at the eastern neighbourhood of Jobar and at the southern ring road, where rebels have overrun roadblocks and army positions, the sources said.
Assad, battling to crush a 22-month-old uprising in which 60,000 people have died, has lost control of large parts of the country but his forces, backed by air power, have so far kept rebels on the fringes of the capital.
Syrian state media said six people, including a woman and three children, were killed by rebel mortar fire on a bus station in the north-eastern district of Qaboun on Thursday, with several other people seriously wounded.
Activists put the overnight death toll in the city at 30 people, mostly from heavy army bombardment on the contested neighbourhoods of Jobar, Zamalka and Hajar al-Aswad.
Damascus residents, long accustomed to the sounds of war, said Wednesday's barrage some of the heaviest they had heard.
"They've gone insane. All of them. They're insane," one central Damascus resident said by telephone.
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.
Despite the sustained rebel challenge, and an Israeli air strike near Damascus last week, Assad has remained defiant, telling a visiting senior Iranian official on Sunday that Syria can confront "current threats... and aggression".
"SHELLS HITTING CIVILIANS"
State media said the army had pushed back rebels from Jobar and other eastern districts. Authorities have banned most independent media from the country, making it difficult to verify events on the ground.
A retired military officer in Damascus said the shelling of rebel areas was hitting civilians and fighters indiscriminately. The army "stand hundreds of metres away and fire shells. And the shells fall on anyone. Women and families and anybody. Where is the courage in that?"
Neither side has gained a clear military advantage in the civil war pitting mostly Sunni Muslim rebels against security forces dominated by Assad's minority Alawite sect.
Alloush said the aim of the latest 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 defence 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 armoured vehicles captured from Assad's forces over the past few months, according to opposition sources.
Many parents took their children out of school early on Wednesday and the acrid, stinging smell of explosives hung in the air, forcing some people to keep their windows closed. (Reporting by Khaled Yacoub Oweis, Amman newsroom; Editing by Pravin Char)