Maj Gen Robert Mood spoke after arriving in the Syrian capital, Damascus, to take charge of an advance team of 16 UN monitors trying to salvage an international peace plan to end the country's 13-month-old crisis.
Under the plan, a ceasefire is supposed to lead to talks between Mr Al Assad and the opposition on a political solution to a conflict that has killed more than 9,000 people.
Maj Gen Mood told reporters that the 300 observers the UN has authorised for the mission "cannot solve all the problems" in Syria, asking for cooperation from forces loyal to Mr Al 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 ceasefire began unravelling 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," Mr Kellenberger told the Swiss newspaper Der Sonntag. He met with Syrian leaders earlier 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 swathes of the city in ruins, before two UN monitors moved into an upscale hotel there last week.
Since then, the level of violence has dropped, although gun battles still frequently break out. "The shooting has not stopped in Homs," local activist Tarek Badrakhan said yesterday.
An amateur video posted online Saturday showed the observers walking through a heavily damaged neighbourhood, where residents collected a body lying in the street and put it in the back of a pickup lorry.
Maj Gen Mood, a Norwegian, was appointed head of the observer mission by UN 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.