The advance party of a 30-strong mission of unarmed observers was set to arrive late on Sunday night and deploy on Monday. But William Hague, the foreign secretary, said there were limits to what the group could achieve given the fragility of the truce established last week.
"This number of people cannot possibly effectively monitor what is happening in the whole country," Hague told Sky News. "The plan will be for a much larger [team], more in the hundreds, of monitors to follow them provided the [ceasefire] plan is being implemented by all concerned," he said.
The observers will have much to inspect. In Homs on Sunday, Walid al-Fares, an activist living in Khalidiya, one of the neighbourhoods where mortar bombs have landed, said: "Early this morning we saw a helicopter and a spotter plane fly overhead. Ten minutes later, there was heavy shelling."
Another resident said that government loyalists were using heavy machine guns to shoot into the area. Rami Abdelrahman, head of the British-based Syrian Observatory for Human Rights, said shells were being fired at a rate of one a minute.
Abdelrahman said there had also been overnight clashes in Aleppo: "People said they heard explosions and shooting after rebels attacked a police station and then clashed with police."
The fresh violence came after the fragile ceasefire had already been rocked by fighting in Homs on Saturday, when regime forces shelled rebel-held neighbourhoods. Rebel fighters were also reported to have fired rocket-propelled grenades at an area held by regime loyalists.
On Saturday in Aleppo, Syria's largest city, regime forces opened fire on mourners at a funeral, while rebel gunmen ambushed a car carrying soldiers in the southern province of Deraa.
The two sides have traded allegations of truce violations since it took effect formally last Thursday. However, Saturday's reports of the use of heavier weapons suggested the ceasefire was in jeopardy.
A video, shot in a destroyed part of what the cameraman says is the Homs area of al-Qarabis, showed two tanks rushing through the streets to the sound of heavy gunfire and explosions. "Look with your own eyes. Look, world. Watch what they are doing," the man making the video screams as a tank raises its turret.
The Syrian state news agency Sana said "armed terrorists" killed five people in ambushes around the country on Saturday and kidnapped a parliamentary candidate from the north.
Since the truce brokered by Arab League and UN envoy Kofi Annan came into effect, fewer deaths have been reported. The Syrian Observatory for Human Rights said the shelling on Saturday in Homs lasted for about an hour and there were no reports of casualties.
The UN has warned it will "consider further steps" against the Assad regime if Syria does not end the violence and comply with Annan's ceasefire and six-point peace plan.
Troops for the peacekeeping mission will be drawn from existing UN military observer missions in the Golan Heights, Lebanon and Sudan; sources said they were ready to fly to Damascus within hours.
The resolution on Saturday is the first to be passed by the security council since the Syrian crisis – which has pushed the country to the brink of civil war – began more than a year ago. Significantly, it won the support of Russia and China, which had controversially vetoed a previous resolution aimed at bringing to an end violence that has claimed 9,000 lives in the past 13 months.
Russia's UN ambassador, Vitaly Churkin, said Moscow was satisfied with the latest western-Arab resolution authorising the deployment of the first batch of unarmed UN observers to Syria to monitor its fragile truce. Speaking after the vote, however, Churkin warned against "destructive attempts of external interference" in Syria.
Welcoming the deployment, Hague had said: "This mission is a vital step in supporting the fragile ceasefire in Syria. It is essential that it begins its work urgently and without impediment. I urge all parties to maintain the ceasefire to allow the monitoring mission to deploy and complete its task."