A spate of car bombings in Damascus on Thursday killed at least 83 people, including more than 60 in an attack near the headquarters of the ruling Baath party, a watchdog said.

The most lethal attack, in central Damascus, was by a suicide bomber who detonated his explosives-laden car near the entrance of the Baath party's main offices, said the Syrian Observatory for Human Rights.

The majority of the 61 people who died in the powerful blast were civilians, including many children from a nearby school, the Observatory said on Friday, adding 17 were members of the security forces.

The attack was the deadliest in Damascus since bombers first began targeting Damascus about a year ago. Twin suicide bombings killed 55 people in the Syrian capital on May 10, 2012.

Another 22 people were killed in an apparently coordinated triple bombing targeting security headquarters in the northern Damascus district of Barzeh on Thursday, including 19 members of the forces, said the Observatory.

Attacks have increasingly targeted government or security buildings in Damascus in recent months, many of them claimed by the jihadist Al-Nusra Front which the United States has designated as a "terrorist" group.

Thursday's attacks were condemned by both the regime and opposition as well as the United States, Russia and UN chief Ban Ki-moon.

Elsewhere in Syria, 38 people were killed Thursday in Daraa, cradle of the anti-regime revolt in the south of the country, including 18 in an air raid on a medical centre, the Britain-based Observatory said.

In the region of Daraa, seven members of one family -- six women and a child -- were killed in an explosion at Tsil, and 10 other civilians died in army bombardments on the town of Jassem.

In the northwestern province of Idlib, where sectarian strife has been rising, 40 Shiites kidnapped by an armed group and 300 Sunnis abducted in reprisals last week were released after mediation talks between families.

The majority of the Syrian population fighting the regime of Bashar al-Assad are Sunni Muslims, while the president and his clan belong to the Alawite branch of Shiite Islam.