Shelling killed two Yemenis in the Old City of Sanaa, designated a World Heritage site by UNESCO, government officials said Wednesday, violating a cease-fire signed by the government and a dissident general backing opposition groups.

Residents said they heard shots ring out across the city, which opposition groups blamed on security forces loyal to President Ali Abdullah Saleh, while the government blamed renegade general Ali Mohsen's forces.

The militants of Ali Mohsen ... shelled the Old City of Sanaa. One of the shells came down on a house, killing two people and injuring three others, a government source said.

The Old City, described by UNESCO as an extraordinary density of rammed earth and burnt brick towers ... associated with the civilizations of the Bible and the Koran, has so far escaped major damage in clashes between the government and opposition groups calling for an end to Saleh's 33-year rule.

The cease-fire was set to take effect Tuesday after a month of armed clashes between Saleh's forces and an opposition composed of protesters, tribesmen and dissident soldiers.

Saleh's refusal on three occasions since unrest erupted in February to accept a power-transfer formula drawn up by Gulf neighbours has aggravated the protests, bringing Yemen to a political standstill and creating a security vacuum beneficial to al Qaeda militants. Central governance has faltered.

In the southern city of Taiz, Yemen's third largest, two people, including a woman, were killed Wednesday from shelling after the truce went into effect. Residents accused government forces of attacking some neighbourhoods in the city.

State media have said the cease-fire was taking hold and a mediator expressed optimism it would remain in place.

Under the accord, both government and opposition forces agreed to dismantle armed checkpoints across the capital and free all people kidnapped during months of anti-Saleh protests.

Tension between government and opposition forces has risen after a U.N. Security Council resolution condemned violence in Yemen and urged Saleh to sign the Gulf initiative to hand over power.

A car bomb exploded in a vehicle belonging to a high-ranking security official in the southern port city of Aden, but the official was not in the car when it went off.

(Reporting By Mohamed Sudam; Writing By Nour Merza; Editing by Mark Heinrich)