UN chief demands safety guarantees for ceasefire monitors amid reports of continued shelling of opposition strongholds.
International peace mediator Kofi Annan is in Qatar to brief the Arab League on a peace plan to end the conflict in Syria.
The UN-Arab League special envoy on Syria is meeting Arab ministers and Nabil el-Arabi, the league’s secretary general.
Continuing violence in Syria has dampened expectations in many quarters for the prospects of a peace plan proposed by Annan, which was supposed to halt all fighting and bring about a withdrawal of Syrian heavy weaponry from areas under attack.
The plan was agreed to by Syrian President Bashar al-Assad, but compliance by Syrian forces appears to have been partial, despite the arrival in Damascus on Sunday of a six-man advance team of UN monitors.
UN officials said the team was still drawing up plans, and the head of the observer team, Moroccan Colonel Ahmed Himmiche, said on Tuesday it would take time to reach badly affected towns and cities.
“There should be co-ordination and planning and we should move … step by step,” he said. “It’s not an easy process.”
The group is to be reinforced by an additional 25 monitors who are expected to arrive in the next few days, he said.
Hopes for the observation mission have been tempered by the failure of a previous Arab League mission which was hampered by regime restrictions on movement, and UN chief Ban Ki-moon has demanded his monitors be given free access.
Russia, one of the Syrian government’s key allies, said on Tuesday that unspecified external forces were seeking to undermine Annan’s efforts, saying support for opposition forces was threatening the ceasefire.
Sergei Lavrov, the Russian foreign minister, suggested armed opponents of Assad’s government were largely to blame for persistent violence that has marred observation of the truce.
“There are those who want Kofi Annan’s plan to fail,” Lavrov said in televised remarks.
“Today, those who … from the beginning foretold the failure of Annan’s plan are doing a lot to see to it that this prophecy comes true.
“They are doing this by delivering arms to the Syrian opposition and stimulating the activity of rebels who continue to attack both government facilities and … civilian facilities on a daily basis,” Lavrov said.
Activist reports of shelling against opposition strongholds by Syrian government forces appeared to deal a fresh blow to the ceasefire.
While the overall level of violence is down since the ceasefire formally took effect on Thursday, the regime has stepped up attacks.
The number of people killed every day has also risen steadily since a brief lull that coincided with the start of the truce. Activists reported at least 26 people killed on Monday.
Army tanks were reported to have shelled the southern town of Busra al-Harir, killing at least two people, according to the the UK-based Syrian Observatory for Human Rights.
The town, about 70km south of the capital of Damascus, is considered a stronghold of anti-government Free Syrian Army fighters.
Government forces also shelled the Khaldiyeh neighbourhood in the central city of Homs, a centre of the rebellion against Syrian President Bashar al-Assad, according to the Observatory.
Areas of Homs have been under continuous attack for weeks, with only a short lull on the first day of the ceasefire, according to activists.