U.N.-Arab League envoy Kofi Annan says he is “encouraged” by reports that a shaky cease-fire between government troops and rebels in Syria seems to be holding, despite scattered violence.

In a briefing on Syria to the U.N. Security Council Thursday, Mr. Annan urged the Syrian government to take further steps by removing troops and heavy weapons from major population centers.

The cease-fire is being watched closely by skeptical Western envoys and Syrian opposition groups who are weighing President Bashar al-Assad's good faith in observing the peace plan brokered by Mr. Annan.

U.N. chief Ban Ki-moon said the international community must be united if it is going to keep Syria from descending into “chaos.” At a news conference in Geneva, he said he hopes to send observers to the country soon.

There were reports of violence after the cease-fire took effect.

Syrian state media say “armed terrorists” bombed a military bus in the city of Aleppo, killing one soldier and wounding 24 officers and cadets.

Rights activists said Syrian forces killed at least three civilians. They say the violence took place in areas including the protest hubs of Homs and Hama.

Opposition groups also said Syrian troops remain deployed in flashpoint cities and are on high alert.

Opposition leaders called for demonstrations to test the resolve of the government to abide by the cease-fire.

The United States and its Western partners have pressed for stronger action against Syria for months but have been hindered by Russian and Chinese opposition to what those two nations call outside interference in Syria.

The Assad government on Thursday urged thousands of Syrians who fled from their homes or took refuge in neighboring countries to return home.

One refugee staying on the Turkish side of the border told Reuters news agency nothing from Mr. Assad's government could be trusted.

“We do not trust in the words of Bashar Assad because he is a liar. His government is lying. All the countries know this.”

Refugee Huseyin Kasif at the Yayladagi Refugee Camp in Turkey also dismissed Mr. Assad's intentions, accusing him of spewing “gibberish.”

“This cease-fire will not stay long. He has been promising this for a year. I do not think he will withdraw neither tanks nor troops. He is lying. Whenever he says he will end the massacre, he kills more.''

Opposition Syrian National Council member Bassam Imadi Thursday said he was not certain the cease-fire would hold. And he said, if it did, it could only be a first step.

“There should be some kind of dialogue. In our mind, the dialogue should be about the transition phase after Assad steps down. So this could be the second step.''

Syrian state-run media said Thursday that 160 armed men from the Damascus and Latakia areas had surrendered to authorities.

President Assad has refused to step down in the face of growing international pressure. In a statement Wednesday, Syria said the army had successfully fought off “armed terrorist groups” and had “reasserted the state's rule across the country.”

U.N. officials say more than 9,000 people have been killed in Syria since the uprising began 13 months ago.