Syrian troops fired on residents of the southern town of Daraa again Tuesday as a military assault on the pro-democracy protest hub continued into a second bloody day, a rights activist said.

"The bullets continue against the people, but we are resisting," activist Abazid Abdullah told AFP by telephone from the town near the Jordanian border.

He said the house of the local cleric, who resigned on Saturday in protest at the harsh crackdown, was "surrounded this morning but he was not in."

At least 25 people were killed Monday in intensive shelling of Daraa, 100 kilometres (62 miles) south of Damascus, by thousands of Syrian troops backed by tanks and snipers, activists and witnesses said.

The Syrian army insisted it had been invited into the town at the request of citizens to hunt "extremist terrorist groups."

Daraa has been the epicentre of protests against President Bashar Al-Assad, who according to some activists has opted for a military solution to crush the demonstrations which began six weeks ago.