Renewed violence in the Yemeni capital killed at least 15 people Thursday as forces loyal to the regime and its opponents shelled each other’s strategic positions from hills surrounding the city, medical and security officials said.

The shelling over Sanaa has terrified residents and emptied the streets, already pockmarked by ground battles between rival forces based in different corners of the capital. A number of shops in a main boulevard were scorched from earlier mortar fire, and oil spots covered the streets after electricity transformers took a hit.

Smoke billowed from opposite edges of the city, as two military officials said rival forces were engaged in an exchange of artillery and mortar shelling from northern and southern hills. It was not clear what was hit by the shelling.

The Republican Guard, a force led by a son of President Ali Abdullah Saleh, has been in control of the south of Sanaa, while defecting military units led by Maj. Gen. Ali Mohsen al-Ahmar, a former Saleh aide who sided with the opposition, hold the city’s north.

Officials said six people were killed in central Sanaa when government forces shelled thousands of protesters with mortar rounds and rocket-propelled grenades. Snipers on rooftops also targeted the protesters in Change Square, the center of Yemen’s eight-month-old uprising, and adjacent streets.

Three bystanders were killed by a mortar shell in Sanaa’s northern Hassaba district, the officials said. The district is home to several of the tribal chiefs who switched sides in March to join the opposition against Saleh’s 33-year rule. The Interior Ministry later said four gunmen among supporters of Saleh were also killed. The rival side said one of its fighters was shot dead and 13 were wounded.

The house of a former defense minister, who has declared his support for the protesters, was also hit by government shells, leaving one of the guards dead, a defecting military official said. The former minister was not harmed.

The latest deaths brought to about 100 the number of people killed in Sanaa and elsewhere in Yemen since Sunday, in the worst bout of bloodshed in months. The deaths shattered hope that a cease-fire negotiated Tuesday could be restored and significantly diminished the chances for a proposal by Yemen’s Gulf Arab neighbors to end the crisis.

— Associated Press