Beirut: At least 25 people were killed in Syria on Monday as regime troops rained down explosives on the central protest city of Homs and opened fire in Damascus, Aleppo and Zabadani, activists said.

Upwards of 17 civilians were killed and dozens wounded as security forces renewed their bombardment of Homs, the Syrian Observatory for Human Rights said.

The Britain-based group said the death toll was likely to rise because many of the wounded had suffered critical injuries.

The shelling, which began at dawn, was mainly targeting the neighbourhoods of Khaldiyeh, Baba Amro, Inshaat and Bab Sbaa, the Observatory said.

Syrian state television said "an explosion killed several terrorists who were preparing bombs in a building in Khaldiyeh."

Opposition groups and activists had reported a "massacre" overnight Friday in Homs, saying that more than 200 civilians were killed during a heavy tank and mortar bombardment by
regime forces. "Nail bombs" were also reported to have been used against civilians.

On Sunday, activists reported more shelling in the city, with at least 23 civilians killed.

Elsewhere, two civilians were killed on Monday when their car came under fire from security forces at Sarghaya, near Damascus, the Observatory said. One was reportedly a child.

It added that in the country's second city of Aleppo in the north, a 45-year-old male passenger aboard a bus was shot dead.

The army also launched an attack on the town of Zabadani, northwest of the capital and near the border with Lebanon, the rights watchdog said, adding that one civilian was killed in the neighbouring town of Madaya.