An opposition group said in an e-mail at least 19 people were killed Tuesday across Syria.
As U.N. monitors try to observe the cease-fire's progress, U.S. ambassador the United Nations Susan Rice said President Bashar Assad's regime has no credibility when it says it wants a peaceful end to the violence that has gripped the country since pro-democracy demonstrations began more than a year ago.
"They have lied to the international community, lied to their own people. And the biggest fabricator of the facts is Assad himself," Rice said Monday. "His representatives are merely doing his bidding and under probably some not insignificant personal duress."
The state-run Syrian Arab News Agency has maintained "armed terrorist groups" were responsible for the violence and aggression by the such groups "hysterically escalated" since the start of the cease-fire Thursday.
The cease-fire agreement was brokered by Kofi Annan, a former U.N. secretary-general who is acting as a U.N.-Arab League envoy to try to bring about an end to the bloodshed. Annan will be in Qatar Tuesday for an Arab League meeting on Syria, CNN said.
Six U.N. observers in Syria will be "liaising with the Syrian government, security forces and the opposition members to establish the monitoring process across the country," said Kieran Dwyer, a spokesman for U.N. peacekeeping missions.
In Moscow, meanwhile, Russian Foreign Minister Sergei Lavrov Tuesday warned against third parties becoming involved in the Syrian crisis, RIA Novosti reported.
"I can't help focusing on the problem of outer influence on the Syrian processes," Lavrov told a news conference. "There are countries, external forces, which are not interested in the success of the U.N. Security Council's efforts [on solving the conflict]."
Representatives of many of Syria's Sunni Arab tribes, meeting in Istanbul, Turkey, Monday, announced the creation of a tribal council and declared their opposition to the Syrian regime and its crackdown on dissent, Today's Zaman reported.
The announcement formalized the Sunni tribes' opposition to the minority Alawite regime of Assad, but didn't include information about the council's size or how it would relate with Syria's other opposition groups.