Syrian warplanes pounded the rebel Jubar district of the capital Damascus with unprecedented ferocity on Tuesday, launching 25 raids, a monitoring group said.
"It's the largest number of air raids to hit Jubar since the beginning of the army offensive on the district" six days ago, said Rami Abdel Rahman, director of the Syrian Observatory for Human Rights.
Jubar is in eastern Damascus and has been in rebel hands for a year.
It is considered a strategic position because it leads onto the adjacent Abbasid Place in central Damascus and also opens onto the key rebel stronghold of Eastern Ghouta in Damascus province.
Government forces have been attacking Jubar since last week, with troops backed by fighters from Lebanon's Shiite Hezbollah movement, a Syrian regime ally.
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The offensive is the biggest against Jubar since rebels seized the neighbourhood in mid-2013.
The district has largely been emptied of civilians by fighting, but Abdel Rahman said "dozens of rebels have been killed" in the regime onslaught, although he had no more precise figures.
The attack has used warplanes, as well as artillery and Iranian ground-to-ground missiles.
Regime soldiers and Hezbollah fighters are currently deployed around the outskirts of the neighbourhood, but have not managed to enter despite the heavy bombardment, Abdel Rahman said.
In mid-August, the army seized the Mleiha area, 10 kilometres (six miles) southeast of Damascus, and it is hoping that a capture of Jubar would allow it to launch a double-pronged assault on Eastern Ghouta.
According to the Observatory, at least 180,000 people have been killed since the beginning of the conflict in March 2011.