GENEVA - The onus is on Syria's government to keep to its promise to observe a ceasefire that took effect on Thursday, 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 (happened) today that they stopped the fighting. We are following it very closely. The world is watching, however, with skeptical eyes since previous promises made by the government of Syria have not been kept," Ban told reporters.
Syrian troops appeared to be holding their fire in the hours after a UN-backed ceasefire took effect at dawn on Thursday, bringing relative calm to rebellious towns they had bombarded heavily in recent days.
"I am urging President Assad to keep his promise and to exercise maximum restraint," said Ban. "This ceasefire process is very fragile - it may be broken any time if, and when, there is another gunshot. Even a small gunshot may give both sides a pretext to engage in another fighting.
"This is a very worrisome," he said.
The main opposition movement said the truce was only being "partially observed" as heavy weapons and government troops remain deployed in cities.
State media reported that a "terrorist" bomb blasted an army bus and killed a senior officer in Aleppo after the truce began. State media also reported a bomb wounding officers near Idlib and a ruling party member shot dead in Deraa in the south.
International mediator Kofi Annan, whose six-point peace plan was accepted by Assad's government, was due to brief the Security Council by video -link from Geneva at 1400GMT.