DAMASCUS  - Syrian forces were locked in fierce gunfights Monday with rebels in one city and shelled another, as the UN chief urged “maximum restraint” after the arrival of observers to oversee a truce aimed at ending 13 months of bloodshed.

President Bashar al-Assad’s forces killed at least 12 civilians and were fighting rebels at Idlib in the northwest, while also shelling the flashpoint central city of Homs, said the Syrian Observatory for Human Rights.

Since a UN-backed ceasefire came into force at dawn Thursday, at least 55 people, mostly civilians, have died in violence that prompted UN chief Ban Ki-moon to urge Syria to ensure the truce does not collapse, the Observatory said.

It said on Monday that 11,117 people have been killed in 13 months of unrest - 7,972 civilians and 3,145 military and gunmen, including fewer than 600 rebel fighters.

In Brussels, Ban called on Damascus to exercise “maximum restraint” and the opposition to “fully cooperate” to cement the shaky ceasefire. An advance team of six international observers arrived in Damascus late on Sunday, the United Nations said.

The delegation - the first of 30 monitors the UN Security Council approved on Saturday - will set up a headquarters and prepare routines to verify a cessation of hostilities. “They’ve arrived and they will start work (on Monday) morning,” UN peacekeeping department spokesman Kieran Dwyer said. “The other monitors in the advance party are still expected in Syria in coming days.”

The remainder would come from around the Middle East and Africa “so we can move people quickly and they are experienced in the region,” he told AFP. Their mission is just one part of the six-point peace plan that Syria agreed with UN-Arab League envoy Kofi Annan.

The former UN chief wants more than 200 observers to be deployed in Syria, but the Security Council has said there would be a full mission only if the violence halts. The United States warned on Monday that heightened violence in Syria threatens the sending of the full mission.

New attacks by government forces “call into question the wisdom and viability” of sending the full force, said the US ambassador to the United Nations, Susan Rice.

The observers were welcomed by Syria, which hoped they would see for themselves the “crimes” committed by “armed terrorist groups,” said the state news agency SANA.

They face a perilous task, with Western nations doubting Assad’s commitment to the ceasefire amid reports his forces have kept battering rebel strongholds and clashed with rebels.

A spike in deadly violence forced the Arab League to end its own Syrian monitoring mission in late January, barely a month after sending observers.

Qatar’s emir, Sheikh Hamad bin Khalifa Al-Thani, told a press conference in Rome on Monday that the chances for success for the Annan peace plans were “no higher than three percent.”

On Monday, security forces killed two civilians when they fired on a car in Hama, while a third passenger later died of wounds, said the Observatory.

In the same province, security forces killed a 16-year-old youth after it opened fire at random in the village of Khatab, said the Observatory.

Elsewhere, fierce gunfights broke out at dawn between forces loyal to Assad and rebels in the northwestern city of Idlib, is said.Four civilians were shot dead by security forces during the gunfight. Security forces also shot dead a civilian in the town of Inkhel, southern Daraa province, where the uprising against Assad’s iron-fisted rule erupted in March 2011.

Regime forces resumed shelling rebel neighbourhoods of Khaldiyeh and Bayyada in the central city of Homs, killing three people, the Observatory said. The authorities on Sunday charged that rebels had “intensified” attacks on security forces and civilians, warning of a response, as state media published a list of alleged acts of violence.

Ban voiced concern over the shelling of Homs. “I am very much concerned about what has happened since yesterday and today,” he said. “It is important, absolutely important, that the Syrian government should take all the measures to keep this cessation of violence.”

China and Russia, which raised earlier reservations over the text of the peacekeepers resolution and had vetoed past resolutions, backed Saturday’s vote at the Security Council that approved the monitoring mission.

Meanwhile, Syrian tribes opposed to Assad said Monday they would join forces in an “Assembly of Tribes,” Syrian opposition members and media reported. “Today we proclaim, as children of all tribes, an Assembly of Tribes,” opposition Syrian National Council (SNC) member Mahmut al-Maslat said in a statement read out on behalf of the assembly.