The head of the U.N. observer mission in Syria on Sunday called on President Bashar Assad and the country’s opposition to stop fighting and allow a tenuous cease-fire to take hold.Maj. Gen. Robert Mood spoke after arriving in the Syrian capital, Damascus, to take charge of an advance team of 16 U.N. monitors trying to salvage an international peace plan to end the country’s 13-month-old crisis. Under the plan, a cease-fire is supposed to lead to talks between Mr. Assad and the opposition on a political solution to a conflict that has killed more than 9,000 people.Gen. Mood told reporters that the 300 observers the U.N. has authorized for the mission "cannot solve all the problems” in Syria, and he asked for cooperation from forces loyal to Mr. Assad as well as rebels seeking to end his rule."We want to have combined efforts focusing on the welfare of the Syrian people,” Gen. Mood said. "True cessation of violence in all its forms.”The cease-fire began unraveling almost as soon as it went into effect April 12. The regime has kept up its attacks on opposition strongholds, while rebel fighters have continued to ambush government security forces. Defying a major truce provision, the Syrian military has failed to withdraw tanks and soldiers from the streets.Despite the violence, the truce still enjoys the support of the international community, largely because it views the plan as the last chance to prevent the country from falling into civil war — in part because it does not want to intervene militarily.