More than 20 people have been killed in blasts targeting security buildings in the city of Idlib in northwestern Syria.

The majority of those killed were members of the security forces, according to the Syrian Observatory for Human Rights.

Reports from Syria cannot be independently verified as state authorities have barred international journalists and rights groups.

Syrian state television put the death toll at eight, among them civilians, and said scores were also injured in two blasts that took place in Idlib's Hananu Square, on Carlton Street.

It said "terrorists" were behind the attacks.

It showed blood stains on the ground, and groups of angry people denouncing the violence and expressing support for the regime of President Bashar al-Assad.

One apartment block appeared in ruins and cars nearby were flattened by the force of the explosion.

A powerful blast, probably a car bomb, was also reported near the capital Damascus, causing casualties.

Overnight, a rocket-propelled grenade hit the Central Bank in the capital, state media reported and on Friday a suicide car bomb in the heart of the capital also killed 11 people.

Anti-regime activists have accused the government of being behind the series of explosions, while authorities say "terrorists" are responsible.

The violence is taking place despite a UN-backed ceasefire that came into effect on 12 April but has failed to fully take hold.

Veteran peacekeeper Major General Robert Mood urged all sides to abide by the ceasefire as he arrived in Damascus on Sunday to take command of the UN military observer mission overseeing the truce.

The peace plan brokered by UN-Arab League envoy Kofi Annan calls for a commitment to stop all armed violence, a daily two-hour humanitarian ceasefire, media access to all areas affected by the fighting, an inclusive Syrian-led political process, a right to demonstrate and the release of arbitrarily detained people.

UN leader Ban Ki-moon has condemned the bomb attacks but said UN observers had brought some improvement in areas where they have been deployed.