Nineteen people have reportedly been killed in shelling by tanks in residential areas in Syria as president Bashar al-Assad attempts to crush anti-government protests, defying calls for an end to the brutal crackdown.

Ammar Qurabi, head of the National Organisation for Human Rights in Syria, said 13 people were killed in the southern village of al-Harah on Wednesday.

Tanks also shelled a residential district in Syria's third largest city Homs and at least five people were killed, a rights campaigner in the city said. A sixth person was killed by a sniper shot to the head as he stood in front of his house.

Most were killed in shelling, but gunfire killed several of the victims, Qurabi said.