Nearly 30 civilians, including children, have been killed by airstrikes in the rebel stronghold of Eastern Ghouta, outside the Syrian capital, Damascus.
The Britain-based watchdog Syrian Observatory for Human Rights, which closely monitors the Syrian civil war, said most of the airstrikes were carried out by Russian jets.
There was no immediate confirmation by the Russian military, which is providing air support to government forces fighting rebels.
A government forces base, the only one in the region, was surrounded by rebels earlier this week. Syrian state television said "army units had launched an assault to break the siege."
Rami Abdel Rahman, head of the Syrian Observatory, said "violent clashes were taking place" Thursday close to the base, which is believed to hold some 250 government soldiers.
Eastern Ghouta, home to 400,000 residents, has been cut off from food and medical aid since 2013.
Last week, the Syrian government allowed Red Cross to evacuate 29 critically ill patients, including 18 children and four women suffering from heart disease, cancer, kidney failure and blood diseases, in addition to cases requiring surgery not available in the besieged area.
The United Nations had called for an emergency evacuation of nearly 500 patients and an end to the siege to allow access for humanitarian medical and food aid.