UN observers acknowledged on Tuesday that they face a "difficult" job firming up a shaky ceasefire in Syria as five civilians were killed in fresh violence as the truce entered its sixth day.

Colonel Ahmed Himmiche, a Moroccan who heads a six-strong advance team preparing for the deployment of a 30-person mission, said the observers would move forward one step at a time.

Secretary of State Hillary Clinton said the United States was still "hoping for the best" but was discussing with other powers what to do in the event the peace plan collapses.

Her Russian counterpart, Sergei Lavrov, pointed the finger at the opposition -- 11 of 35 people killed in violence on Monday were soldiers -- and called on its foreign supporters to press the rebels to honour the hard-won truce.

UN chief Ban Ki-Moon called on regime forces to exercise "maximum restraint" and the opposition to "fully cooperate."

Three of the five dead on Tuesday were killed in regime shelling of Idlib, a northwestern province close to the Turkish border, where there is a strong presence of rebel fighters, the Syrian Observatory for Human Rights said.

Two more were killed and dozens wounded in a bombardment of the Basr al-Harir district of Daraa province, south of Damascus and cradle of the 13-month-old uprising against President Bashar al-Assad's regime, the Britain-based watchdog said.

The rebel Khaldiyeh and Bayada districts of the flashpoint central city of Homs also came under renewed shelling, it added.

The opposition Syrian National Council accused the regime of "flagrant violations of the ceasefire" and called on the UN observers to "travel to Idlib and Homs immediately to see first-hand the massacres which the regime is carrying out and has not stopped carrying out."

UN and Arab League envoy Kofi Annan, who brokered the six-point peace plan, was to travel to Qatar for a ministerial meeting of the Arab League on the crisis later on Tuesday, his spokesman said.

Colonel Himmiche said "it's a difficult mission that needs coordination and planning."

"No ceasefire, not even the beginnings of a political process -- this mission will be one of the toughest ever undertaken by the United Nations," he added.

Clinton called on Damascus to honour Annan's plan in full, not just the promised ceasefire.

"What the Assad regime needs to do is to make clear that they're going to silence their guns, withdraw their troops and work toward fulfilling the six-point plan," she said.

Complying with the Annan plan also means allowing peaceful demonstrations, releasing political prisoners and allowing a peaceful political transition to begin, Clinton added.

"We want to see a political process begin, but if violence is renewed, the regime reverts to shelling its own people and causing a great deal of death and injury, then we're going to have to get back to planning what our next steps (are)."

State Department spokesman Mark Toner also pressed Syrian authorities to comply with the other points, including releasing prisoners.

"There's no movement on any of the other five points and it appears that the fragile ceasefire is eroding as well," Toner told reporters.

French Foreign Minister Alain Juppe said "stronger sanctions" against Damascus must be adopted to "pressure the Syrian regime" and erode its resources.

Damascus ally Moscow took aim, without naming them, at supporters of the rebels, such as Qatar and Saudi Arabia, for what it acknowledged was a "fragile" truce.

"There are countries -- there are outside forces - that are not interested in the success of current UN Security Council efforts," Lavrov said.

The advance team of military observers arrived in Damascus late on Sunday.

The delegation is setting up a headquarters and preparing routines to verify the ceasefire, a spokesman said.

Russia, which voted in favour of the text after vetoing two previous draft resolutions, will be "substantially" represented in the mission, Deputy Foreign Minister Sergei Ryabkov said.

"The specifics of our participation in the observers mission are being worked on right now," Interfax quoted Ryabkov as saying.

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.

The UN chief voiced concern late Monday.

"I am very much concerned about what has happened since yesterday and today," Ban said.

"It is important, absolutely important, that the Syrian government should take all the measures to keep this cessation of violence," he said, adding that Damascus must "guarantee" free access countrywide to the military observers.

Meanwhile, Syria's Foreign Minister Walid Muallem will visit China on Wednesday, Beijing announced, to showcase efforts taken by Damascus to execute the UN ceasefire.