While fighting raged in Syria's commercial capital Aleppo for a fourth straight day on Tuesday, clashes between the Syrian army and rebels also erupted in Damascus and other parts of the country, a watchdog said.

The battles in the capital were focused in the southern districts of Tadamun, Kazaaz and the Palestinian refugee camp Yarmuk, and were sparked when rebels firing rocket-propelled grenades at military checkpoints, the Syrian Observatory for Human Rights said.

Residents two weeks ago fled Tadamun and the Yarmuk camp in the face of a large scale offensive by regime troops to "cleanse" Damascus of rebels before the start of Muslim holy month of Ramadan.

On the outskirts of the capital, clashes erupted after midnight on the road to Halbun, while shelling was reported in nearby Qudssaya, and in New Artuz, the Britain-based Observatory reported.

For its part, the Syrian Revolution General Council reported raids by regime troops in the Damascus district of Jubar and that military checkpoints had been set up in several areas of the city.

Local Coordination Committees, made up of activists on the ground, reported shelling by helicopter gunships of the Zamalka district -- adjacent to Jubar -- throughout the morning.

Elsewhere in Syria, clashes erupted in multiple districts of eastern Deir Ezzor city, including near a police station, while one civilian was killed by sniper fire, the Observatory said.

In the southern province of Daraa, a camp for displaced persons came under shelling by regime troops, who battled for hours with opposition fighters.

The town of Tafas and al-Ghariyeh also came under shelling, leaving an unknown number of casualties, while in Hara, regime forces arrested six people.

Clashes and the sound of explosions were reported at the University of Idlib and another neighbourhood in the northwest city.

In Aleppo province, a defected soldier was killed in clashes with the army, while fighting raged in several districts of Aleppo city, including on the outskirts of the rebel-stronghold of Salaheddin and near a police station in Salhin.

The Observatory said violence across the country on Monday saw 93 people killed -- 41 civilians, 19 rebels and 33 soldiers.

More than 20,000 people have been killed in Syria since the start of the revolt in March 2011, according to the Observatory. This number is impossible to independently verify.