A car bomb has exploded near the headquarters of Syria's ruling Baath party in central Damascus killing at least 31 people, while mortars shelled the capital for the second successive day.

In a separate incident in Dera'a province, south of Damascus, government jets bombarded a field hospital killing 18, including rebel fighters, paramedics, a child, his father and five unidentified people.

The Damascus bomb went off at a checkpoint in the Mazraa neighbourhood and was blamed by state media on "terrorists" - the regime's definition for rebels.

"It was huge. Everything in the shop turned upside down,'' a local resident said.

At least 10 people were injured and casualties were mostly among civilians, UK-based Syrian Observatory for Human Rights said.

Cars as far as 300 metres away from the blast were badly damaged.

Syria's state media said the blast was set by a suicide bomber. Suicide attacks are often used by the al-Qaeda-linked rebel group Jabhat al-Nusra.

Fighting in Damascus has been raging for months on Damascus' suburbs but have so far been kept away from the city centre. Rebels control districts in the south and east of the capital.

Two shells exploded near the Syrian Army General Command in central Damascus. There were no reports of casualities.

President Bashar al-Assad's palace was hit recently by two shells causing material damage. A football player was killed by a shell exploding over a stadium.

About 70,000 people have lost their lives in the two-year conflict, according to the United Nations.