Beirut: A barrage of gunfire by security forces that left residents cowering in their homes killed at least six people and wounded several others on Wednesday in Homs, a hotbed of opposition to President Bashar Assad's autocratic regime, activists and residents said.

For days, security forces have been pursuing activists and anti-government protesters in Homs, part of a broader and ferocious crackdown to try to crush the most serious challenge to the 40-year Assad dynasty.

The UN says more than 2,200 people have died in nearly six months of protests.

"All through the night, there was shooting. The gunfire didn't stop," said a resident of the city.

"I can't tell exactly what is going on because it's dangerous to go out."

He spoke on condition of anonymity for fear of reprisals. Omar Idilbi, a spokesman for the Local Coordination Committees, an activist network, said security forces simultaneously stormed several districts in the old part of the city, including the Bab Dreib, Bab Houd and the Bayada neighbourhoods.

Six people were confirmed dead in ongoing shooting in those areas, he said.

The London-based Observatory for Human Rights, which has a network of activists across the country, said seven were killed.

Homs, Syria's third largest city has seen some of the largest anti-regime protests in Syria over the past months, despite repeated crackdowns.