Hundreds of Syrian troops backed by armour early on Monday have moved into the flashpoint southern Syrian town of Deraa where heavy shooting was heard, according to witnesses on the scene.

"Hundreds of security service men entered the town, accompanied by tanks and armoured vehicles," Abdullah Al-Harriri, an activist, told the AFP news agency in Nicosia by telephone.

"The men are firing in all directions and advancing behind the armour which is protecting them," he added. "Electricity is cut off and telephone communications are virtually impossible."

Footage aired by an opposition news organisation on Monday, transmitted via satellite, appeared to show Syrian military firing at unseen targets with sniper rifles. Al Jazeera is unable to verify the veracity of the footage.

Security forces loyal to Bashar al-Assad, the country’s president, stormed the large Damascus suburb of Douma early on Monday, shooting at unarmed civilians and arresting residents, rights campaigners said.

"There are injured people. Scores have been arrested. The security are repeating the same pattern in all the centres of the democratic uprising. They want to put down the revolution using the utmost brutality," the unidentified rights campaigner told the Reuters news agency, from Damascus.

Al Jazeera's Rula Amin, reporting from Damascus, said that the events on Monday marked a change in methods by security forces.

Up until now, she said, security forces have cracked down in reaction to protests. But the flood of troops into Douma and Deraa came in the absence of any demonstrations. 

"Today, we're seeing a different tactic with security forces sweeping the towns," she said, noting reports of house-to-house searches, arrests and random shooting coming from both towns. 

Communications were cut off and, for the first time, the military has become directly involved in quelling the uprising, much to the disappointment of opposition activists.

"They were hoping the army would not get involved," Amin reported. "They feel this is only the beginning of a very serious crackdown."

Douma has been the site of many large protests since the uprising against Assad began in Syria earlier in the year.

The current protests against the regime started in Deraa where dozens of Syrians have been killed in the violent repression of demonstrations by the security forces.

Thousands of residents of Deraa province buried several victims of the repression after prayers on Sunday.

A demonstration followed, but the security forces did not intervene, an activist said. Asking to remain anonymous, he said the protesters brandished Syrian flags and placards calling for "suppression of Article 8 of the constitution" on the supremacy of the Baath Party.

Most shops stayed closed in a sign of mourning.

The fighting on Monday came after the killing of civilians by security forces in the coastal town of Jableh the previous day.  

At least 13 people had been killed in Jableh, the Syrian Observatory for Human Rights said on Monday.

At least 352 people have been killed in Syria since protests began on March 15, according to figures compiled by AFP.

And Wissam Tarif, executive director of INSAN, a Syrian human rights group, said that according to the organisation's most recent count on Friday there were 221 "forcefully disappeared people" in Syria.

Al Jazeera