Syrian regime forces were engaged in heavy fighting with rebels in southern areas of opposition-held Eastern Ghouta on Sunday, a day after a UN call for a ceasefire, a monitor said.
The clashes on the edges of Eastern Ghouta killed at least 13 members of pro-regime forces and six fighters from the Jaish al-Islam rebel group, the Britain-based Syrian Observatory for Human Rights said.
"Violent clashes are taking place in the Al-Marj area, which is a frontline" of the rebel-held enclave near Damascus, Observatory head Rami Abdel Rahman said.
"They are the most violent clashes to take place since the beginning of the month," he added.
Mohamed Alloush, a key figure in the Jaish al-Islam, tweeted that the rebels were "resisting" against bids by regime forces to enter the region.
More than 500 civilians have been killed in a bombing campaign launched a week ago by regime forces on Eastern Ghouta.
According to the Observatory and a Syrian pro-regime newspaper, the clashes are aimed at paving the way for a ground offensive by government forces against Eastern Ghouta.
The clashes came a day after the UN Security Council unanimously adopted a resolution calling for a 30-day ceasefire in Syria "without delay" to allow for aid deliveries and medical evacuations.