The onus is on Syria's government to keep to its promise to observe a ceasefire that took effect today, UN Secretary General Ban Ki-moon said.
He told a news conference in Geneva that the situation in Syria looked calmer and he was working with the UN Security Council to send a team of truce observers as quickly as possible, once there is a mandate from world powers.
But he warned that the international community must remain unified to avoid Syria descending into chaos."I really like to see this ceasefire continue to be sustained. It just today that they have stopped the fighting. We are following it very closely. The world is watching, however, with sceptical eyes since previous promises made by the government of Syria have not been kept," Mr Ban told reporters.
Syrian troops held their fire today after a UN-backed ceasefire deadline passed, giving respite to rebellious towns that have suffered heavy bombardments, but international envoy Kofi Annan pressed Damascus to pull its forces back entirely.
"Syria is apparently experiencing a rare moment of calm on the ground," Mr Annan said in a statement as he briefed a United Nations Security Council which has been divided along Cold War lines by the 13-month crisis at the heart of the Middle East.
Western leaders, who share Syrian dissidents' doubts about Dr Assad's willingness to engage in a democratic process that would end his family's four decades of absolute power, stepped up calls on Russia and China, veto-wielding bulwarks for Syria in the Security Council, to extend their support for Annan's six-point peace plan by joining a united push for negotiations.
Echoing reports from weary opposition activists in some of the most battered districts of Homs, Hama and Idlib, Mr Annan said: "The cessation of hostilities appears to be holding ... This is bringing much-needed relief and hope to the Syrian people who have suffered so much for so long in this brutal conflict."
But he urged the Security Council to demand a full military withdrawal to bolster the extremely fragile truce.
President Bashar al-Assad's tanks and infantry are still deployed in cities in defiance of Mr Annan's plan, state media are accusing "terrorists" of attacks on troops that might justify retaliation, and rebels are urging demonstrators to test the ceasefire by massing on the streets tomorrow.
A report on state media that a "terrorist" bomb blasted an army bus and killed a senior officer in Aleppo after the truce began raised a possibility troops will keep a pledge to hit back. State media also reported a bomb wounding officers near Idlib and a ruling party member shot dead in Deraa in the south.
Diplomats at the United Nations in New York, who heard Mr Annan brief the 15-nation world policy body by videolink from Geneva, said the Council should swiftly mandate the despatch of some 200 unarmed peace observers to Syria to monitor the truce.
French president Nicolas Sarkozy and US president Barack Obama called on Syria to abide fully with a UN-backed plan to stop a "brutal crackdown" and vowed to intensify efforts to end the violence, including steps at the UN Security Council.
In a statement issued after a conference call between the two men, the leaders also urged Iran to undertake serious negotiations when major powers begin talks with the Islamic Republic in Istanbul tomorrow.
"The two presidents urged the Syrian regime to respect scrupulously and without preconditions its commitments to the plan of the Arab League-UN envoy. The regime will be judged by its acts," the statement said.