Syrian troops backed by tanks have stormed a Damascus suburb, activists said as an advance team of UN ceasefire observers continue their tour of flashpoint areas to lay the groundwork for an expanded monitoring mission.

Activists said that explosions shook the town of Douma early on Sunday as soldiers launched an operation there to crush an armed rebellion against President Bashar al-Assad.

"Regime forces backed by tanks stormed Douma under heavy gunfire," Damascus Revolutionary Council, an activist network, said in a statement.

Activist video posted online showed columns of smoke billowing into the sky over Douma, 10km north of Damascus. Three people have reportedly been killed there.

The reports of violence comes as Kofi Annan, the Arab-UN envoy, asked the Assad government and the opposition to immediately halt violence and the use of heavy weapons.

"I urge all forces whether governmental, opposition or others to put down their weapons and work with the United Nations monitors to consolidate the fragile cessation of violence in all its forms," Annan said in a statement.

"The government in particular must desist from the use of heavy weapons and, as it has committed, withdraw such weapons and armed units from population centers and implement fully its commitments under the six-point plan."

Annan welcomed a UN plan to expand monitoring mission to Syria, saying: "The work of the mission should help create the conditions conducive to launching the much-needed political process, which would address the legitimate concerns and aspirations of the Syrian people."

The UN Security Council approved a resolution to deploy 300 ceasefire observers, mandated to monitor an April 12 truce mediated by Annan.

A smaller team is already in the country and has been touring flashpoints. Violence seems to stops when they are present.

Monitors deployed

The team on Sunday visited the city of Hama and the town of Rastan in Homs province after visiting the main Homs city a day earlier.

The team’s spokesperson said two observers were left behind in Homs to monitor the situation there after five observers toured rebel-held areas.

"Yesterday, the UN advance team visited Homs where they met with the local authorities and all the parties," Neeraj Singh said on Sunday.

"The team drove or walked around the city of Homs and stopped at different locations to talk to the people.

"Following the visit, two UN military observers have now been stationed at Homs since yesterday evening."

Homs has been the center of the uprising that has been battered by tank and mortar shells for weeks.

Their visit in the central city included a stop in Baba Amr, a rebel stronghold battered by a month-long army bombardment that killed hundreds, according to monitors, before it was retaken on March 1.

Video uploaded to YouTube showed at least four observers meeting with activists, who begged them to stay in the city.

"Today is the first day since two months, exactly since 5 February... in Homs without shelling... without killing, without fire," one activist said in the footage.

"Because of that, we want you to stay. Please stay. This is what we want. When you come, shelling stops. When you come, killing stops," he told the observers, who wore blue helmets and bullet-proof vests marked "UN".

The visit came as activists in Homs said the situation in the city was calm.

Singh said the number of observers in the advance team stands at eight, adding however that "we are expecting at least two more military observers to arrive on Monday."

Annan’s six-point initiative, accepted by the government and the opposition, calls for ending the violence, bringing in relief, and forging a political process to address the conflict.

The Russian and European drafted resolution said that deployment of the observer mission, which will be called UNSMIS, will be "subject to assessment by the Secretary-General [Ban Ki-moon] of relevant developments on the ground, including the cessation of violence".

There is no threat of sanctions in the resolution but the council said it will assess the implementation of the mission and "consider further steps as appropriate".

PHOTO CAPTION

Bashar Ja'afari (L), Syria's ambassador to the United Nations speaks with Russian ambassador to the United Nations Vitaly Churkin before a Security Council meeting at the United Nations in New York April 21, 2012.

Aljazeera