Syria's southern town of Daraa came under sustained gunfire from troops Tuesday as a military assault on the epicentre of pro-democracy protests raged into a second day, a rights activist said.

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

Washington, meanwhile, ordered non-essential staff of its Syrian embassy to leave as it also considered imposing "targeted sanctions' on Damascus, which has been shaken by six weeks of protests against President Bashar Al-Assad's autocratic rule.

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

Daraa is one of the main hubs of protests against Assad, who according to prominent activist Rami Abdel Rahman of London-based Syrian Observatory for Human Rights has opted for a "military solution" to crush the demonstrations.

Monday's assault began at dawn with 3,000 to 5,000 army and security forces swooping on Daraa, with tanks taking up position in the town centre and snipers deploying on rooftops, activists said.

The operation came less than a week after Assad signed a decree to abolish nearly five decades of draconian emergency rule in a bid to pacify protesters demanding political reform and the fall of the regime.

Activist Abazid told AFP Syrian forces pounded Daraa with heavy artillery and that "at least 25 martyrs have fallen" Monday.

A group of activists said in a statement "more than 25 people fell but no one could reach them because of the heavy shelling" and that only seven bodies were retrieved, including a father and his two sons.

The Syrian army disputed these reports, with a military official saying the troops entered Daraa "in response to calls for help from" citizens to rid them of "extremist terrorist groups" behind a spate of killings and sabotage.

The official, quoted on state television, said troops backed by security forces clashed with gunmen and the confrontations led to "a number of martyrs in the ranks of the army and the security forces".

A huge crackdown was also reported in Douma, a large suburb in northern Damascus, and nearby Al-Maadamiyeh, said activists and residents reached by telephone.

Some 390 people have been killed in security crackdowns since the protests erupted, rights activists and witnesses say.

Amnesty International said tanks were used to shell civilian buildings in Daraa, commenting: "The Syrian government's brutal reaction to its people's demand for change has reached a new and outrageous low.

"President Bashar Al-Assad must call a stop to this now. He must pull back his army from Daraa immediately and ensure that basic services to the city are restored," said Malcolm Smart, Amnesty International's Middle East and North Africa Director.

The United States ordered its non-essential staff to be evacuated out of Syria.

Any US citizens who remain in Syria should limit nonessential travel "given the uncertainty and volatility of the current situation," the State Department said, adding "embassy operations will continue to the extent possible under the constraints of an evolving security situation."

Washington, which has repeatedly condemned Syria's repression, was also considering imposing "targeted sanctions" against Damascus, National Security Council spokesman Tommy Vietor said.

Assad signed decrees on Thursday scrapping repressive emergency rule, abolishing the state security court that has tried scores of regime opponents, and one to "regulate" protests.

A day later, tens of thousands of people swarmed cities and towns to test the government's sincerity, but scores were killed by security forces, activists and rights groups have said.

Britain, France, Germany and Portugal were meanwhile seeking a UN Security Council condemnation of the violence and an independent probe into the killings.