BEIRUT (Reuters) - Syrian government forces fired rockets at a rebel-held area on Damascus’s outskirts on Sunday, pressing an attack that began the day before and has killed up to 16 people, a medical worker and war monitors said.

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The medical worker and the Syrian Observatory for Human Rights said it was the biggest attack on the Qaboun area, to the city’s northeast, in at least two years.

At least three shells hit government-held areas closer to the center of Damascus and near Qaboun on Sunday, but there were no reports of casualties, a Hezbollah-run military media unit and a Reuters witness said.

There was no immediate government comment on the Damascus fighting.

The Observatory said 16 people had died in the violence around Qaboun since Saturday, the highest death toll from fighting there for more than two years.

The medical worker in nearby Eastern Ghouta, just outside Damascus, said at least 13 people had died. He said he could hear explosions coming from Qaboun early on Sunday.

Witnesses said hundreds of families fled their homes in Qaboun, rebel held parts of Harasta and Barza that were shelled by the army and sought refuge in Eastern Ghouta.

Rescue workers said army shelling also hit Douma, the main rebel stronghold in Eastern Ghouta with at least two dead.

Violence in western Syria has increasingly tarnished a shaky ceasefire that took effect on Dec. 30 with the support of Russia, which backs the government of President Bashar al-Assad, and Turkey, which supports some rebel groups.

Separately, heavy clashes continued in the southern border city of Deraa, where rebels said they had made progress in a week-long offensive where they hope to seize the strategic district of Manshiya.

Rebels said their capture of the district would thwart repeated army attempts to rupture supply lines linking rebel-held areas to the east and west of the city.

The army has also been trying to recapture a main border crossing with Jordan that has been in rebel hands near Manshiya.

Russian and Syrian army jets on Sunday attacked the rebel-held border-town of Nasib, near Deraa, killing at least six civilians, a rebel spokesman and a medic said.

Air strikes have intensified in the last few days on towns to the west, north and east of Deraa. Residents said the attacks had put out of action several field hospitals and a major storage tank providing water in rebel-held territory.