Syrian regime forces on Thursday battled to reach troops trapped in rebel bastion Eastern Ghouta, as a monitor said at least 28 civilians were killed in Russian and government bombardments.
State television said "army units had launched an assault to break the siege" of the Armoured Vehicle Base where some 250 soldiers are believed to be cut off.
On the outskirts of Damascus, Eastern Ghouta is one of the last remaining opposition strongholds in Syria and has itself been under government siege since 2013, causing severe food and medicine shortages for up to 400,000 residents.
The regime base on the edge of the region was surrounded by rebels at the start of the week after an offensive that involved Hayat Tahrir al-Sham, an alliance dominated by a former Al-Qaeda affiliate, the British-based Syrian Observatory for Human Rights said.
Observatory head Rami Abdel Rahman said "violent clashes were taking place" Thursday close to the base, the only one in Eastern Ghouta still held by President Bashar al-Assad's forces.
Ahead of the government operation, Abdel Rahman said at least 29 civilians were killed in Eastern Ghouta Wednesday by Russian and regime bombardments.
Twenty were killed in Russian air strikes in the town of Misraba, while the remainder died in regime raids and shelling in other areas.
Seven children and 11 women were among those killed, said the head of the war monitor, which relies on a network of sources inside Syria.
Victims were taken to a hospital in Douma, where an AFP correspondent saw rescuers bringing in mostly women and children.
Medical staff tried to revive a child who had been pulled from the rubble, but without success. A young girl among the wounded received stitches for a serious injury to her face.
A medical source at the hospital told AFP: "Among the wounded were two women in their 20s. One of them lost both eyes and the other lost one eye."
Eastern Ghouta is one of four "de-escalation zones" agreed by Russia, as well as regime backer Iran and rebel supporter Turkey, to help halt fighting around Syria.
The deal excludes Hayat Tahrir al-Sham, but other larger rebel groups in Eastern Ghouta are part of it.
The latest fighting there comes as regime troops backed up by Russian airpower battle rebels and jihadists on the edge of northwestern Idlib province, the only one still fully beyond government control.
The government push near Idlib -- also a "de-escalation zone" -- follows two months of sporadic fighting that the United Nations says has displaced more than 60,000 people.
Meanwhile, Russia's defence ministry on Thursday said two servicemen were killed in a New Year's Eve mortar attack by Islamist militants on its Hmeimim airbase in Latakia province, but denied media reports seven military planes were destroyed.
Moscow has declared its mission in Syria largely completed after a two-year intervention that has shifted the conflict firmly in Assad's favour.
Russia says it has carried out a partial withdrawal but it will still keep soldiers and bases in Syria.
The war in Syria has killed more than 340,000 people and displaced millions from their homes since it began in 2011 with the brutal repression of anti-government protests.