* Government air strikes back ground forces in rebel-held suburbs
* Activists say first air raids in Damascus since before gas attack
* Assad opponents say shows he no longer fears imminent U.S. strike
By Khaled Yacoub Oweis
AMMAN, Sept 10 (Reuters) - Syrian warplanes bombed rebel suburbs of Damascus on Tuesday for the first time in three weeks, in an offensive that opposition activists said showed President Bashar al-Assad no longer feared attack by the United States.
Not seen in action around the capital since before Aug. 21, when hundreds of people were killed in a poison gas attack that Western powers blame on Assad, government jets mounted attacks on three areas, some in support of assaults on the ground.
As world leaders discussed a Russian proposal to confiscate Syria's chemical weapons and avert U.S. and French action, some of the heaviest fighting was in Barzeh, just north of central Damascus, where residents and opposition activists said air strikes and tank fire supported thrusts by pro-Assad militia.
The Syrian state news agency said troops "inflicted casualties on terrorists" in Barzeh and neighbouring Qaboun.
"Even if the Russian initiative fails, the regime has at least bought itself time," opposition activist Salah Mohammad said. "It seems to be calculating that no strike is coming soon."
Mohammad said jets had staged three air raids on Barzeh on Tuesday while tanks on the heights of Qasioun mountain and in the government-held city centre shelled the area in support of attacks by shabbiha militiamen pushing in to Barzeh from Ish al-Warwar, a district dominated by Assad's minority Alawite sect.
"The fighting is heavy on that front," Mohammed said. "The streets are narrow and tanks cannot be deployed."
A woman living in Damascus said she could see shells hitting Barzeh, apparently fired from Qasioun. Speaking anonymously for fear of reprisals, she said: "Since chemical weapons are a red line, it seems the world has decided it is OK for Assad to destroy Damascus with conventional weapons."
To the southwest, some 15 km (10 miles) from the centre of the capital, aircraft also struck Mouadamiya, one of the places where people were killed by poison gas in an incident for which the government has blamed rebel forces. Washington and its allies say Assad's artillery fired chemical weapons that day.
However, their threats to punish Assad with military action have run into resistance from voters at home, as well as Russian opposition, raising the prospect of a compromise deal.
"By sending the planes back, the regime is sending a message that it no longer feels international pressure on it," opposition activist Wasim al-Ahmad told Reuters from Mouadamiya.
East of the centre, the suburb of Deir Salman also came under air and artillery attack on Tuesday, activist Maamoun al-Ghoutani said. At least two rebel fighters were killed. (Editing by Alastair Macdonald)