Syrian tanks and troops move into Deraa

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Protests in Syria have cost 350 lives, according to rights groups. Photograph: Reuters TV/Reuters

At least five people are reported dead as troops backed by armoured vehicles and tanks moved into the southern Syrian town of Deraa, a centre of the rebellion against the regime of the president, Bashar al-Assad.

Witnesses also told of snipers firing from rooftops in an escalating crackdown on protests that also saw attacks in the coastal town of Jabla and a dawn raid in the Damascus suburb of Douma. There were fears of more violence in another southern town, Nawa.

Syria has closed its southern land border with Jordan, according to Jordanian officials. An official told Reuters the timing was "related to what appears to be a major security operation that is taking place right now".

Thousands of troops, backed by armour, appeared to be making pre-emptive strikes in areas where there were no continuing demonstrations. Until now, the regime's military and police operations have been staged in response to protests.

It has been increasingly difficult to verify claims relating to the violence since Syria has banned most foreign media and sought to impose communication blackouts in areas most affected by the uprising.

A witness in Deraa said: "We need international intervention. We need countries to help us." He added that he had seen five corpses after security forces opened fire on a car.

Another witness said people were using mosque loudspeakers in Deraa to summon doctors to help the wounded as busloads of security forces and troops conducted house-to-house searches, causing panic in the streets.

"They are entering houses. They are searching the houses," he said. "They are carrying knives and guns."

In Jabla, a witness reported the presence of security forces in camouflage uniforms, some with their faces covered, and masked gunmen dressed in black. There were 13 deaths there on Sunday according to the Syrian Observatory for Human Rights.

"Jabla is surrounded by security forces," the witness said, speaking by telephone. "The dead are in the mosques and the houses. We can't get them out."

There was also violence in Douma, a suburb of the capital, Damascus, where authorities were making sweeping arrests. "The security has been heavy here since Friday, but overnight they cut off parts of the city from each other," a witness told AP. "They set up checkpoints to separate the quarters."

A human rights campaigner in the capital told Reuters: "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."

More than 350 people have been killed since the unrest began in Syria five weeks ago. The worst single day for casualties was Friday, with 112 deaths, according to human rights groups. On Sunday, 13 people died in Jabla as security forces moved into a Sunni quarter after protests, according to the Syrian Observatory for Human Rights.

Syrian state TV said nine members of the security forces had been killed in the violence of recent days, seven in clashes with "armed gangs" in Nawa. On Monday, there were reports of bulldozers and military vehicles heading there.

Thousands of people in the town called for the overthrow of Assad on Sunday at a funeral for protesters killed by security forces. Electricity and communications were cut off in parts of the town and residents, some armed, erected barriers in the streets. "Long live Syria. Down with Bashar!" mourners chanted during the funeral. "Leave, leave! The people want the overthrow of the regime."

Meanwhile, Syrian writers issued a declaration denouncing the crackdown. It was signed by 102 writers and journalists, in Syria and in exile, representing all the country's main sects. It called on Syrian intellectuals "who have not broken the barrier of fear" to make a clear stand.

"We condemn the violent, oppressive practices of the Syrian regime against the protesters and mourn the martyrs of the uprising," it said.