BEIRUT, Lebanon - The head of the U.N. observer mission in Syria on Sunday called on President Bashar al-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 Assad and the opposition on a political solution to a conflict that has killed more than 9,000 people.

On Sunday, at least 25 people were killed, including 14 civilians shot dead by troops in central Syria and three soldiers killed in a clash with army defectors, activists said.

Mood told reporters that the 300 observers the U.N. has authorized for the mission "cannot solve all the problems" in Syria, asking for cooperation from forces loyal to Assad as well as rebels seeking to end his rule.

"We want to have combined efforts focusing on the welfare of the Syrian people," he 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 city 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 - and because it does not want to intervene militarily.

Jakob Kellenberger, president of the International Committee of the Red Cross, said that while he is still hopeful, "unfortunately, I am also aware how much this plan is at risk."

"That's why it's especially important for this mission to expand quickly," Kellenberger told the Swiss newspaper Der Sonntag. He met with Syrian leaders this month.

Most analysts say the plan has little chance of succeeding, though it could temporarily bring down the level of daily violence.

That has largely been the case in Homs, Syria's third-largest city, which has emerged as the heart of the uprising. Regime forces pounded parts of Homs for months, leaving large swaths of the city in ruins, before two U.N. monitors moved into an upscale hotel there last week.

Since then, the level of violence has dropped, although gunbattles still frequently break out. "The shooting has not stopped in Homs," local activist Tarek Badrakhan said Sunday.

An amateur video posted online Saturday showed the observers walking through a heavily damaged neighborhood, where residents collected a body lying in the street and put it in the back of a pickup truck.

Mood, a Norwegian, was appointed head of the observer mission by U.N. chief Ban Ki-moon. One hundred monitors should be in the country by mid-May, said mission spokesman Neeraj Singh. It is unclear when or if the full contingent of 300 monitors will deploy to Syria.

Mood brings a wealth of Middle East experience to the job, including stints with U.N. peacekeepers in Lebanon in 1989-90 and as the head of a U.N. peacekeeping mission known as UNTSO from 2009-11. That mission was the U.N's first peacekeeping operation, starting after the 1948 Arab-Israeli war to monitor a cease-fire.