BEIRUT, Lebanon - Five unarmed U.N. truce monitors toured the battered city at the heart of the Syrian uprising on foot Saturday, encountering unusually calm streets after weeks of shelling as a throng of residents clamored for foreign military help to oust President Bashar al-Assad.

Their foray into a chaotic crowd in the city of Homs highlighted the risks faced by the observers, protected only by bright blue helmets and bulletproof vests. It came as the U.N. Security Council voted Saturday to expand the mission to 300 members in hopes of salvaging an international peace plan marred by continued fighting between the military and opposition rebels.

The observers, members of a seven-member advance team that has been on the ground a week, were seen on amateur video Saturday walking through rubble-strewn deserted streets lined by gutted apartment buildings. Activists reported only sporadic gunfire, but no shelling, and said troops had pulled armored vehicles off the streets.

The mission approved Saturday, initially for 90 days, is meant to shore up a cease-fire that officially took effect 10 days ago, but has failed to halt violence. U.N. chief Ban Ki-moon has accused Assad of violating the truce, and said Saturday that "the gross violations of the fundamental rights of the Syrian people must stop at once." Rebel fighters have also kept up attacks.

It is the first time the Security Council authorized unarmed U.N. military observers to go into a conflict area. Saturday's resolution gave Ban the final say on when to deploy them, based on his assessment of the situation.

A previous observer team, dispatched by the Arab League at the start of the year, withdrew after a month, unable to halt the fighting.

Western diplomats put the onus on Syria to make the mission work. The U.S. ambassador, Susan Rice, warned that the United States would pursue sanctions if Assad doesn't comply. Britain's envoy, Mark Lyall Grant, said that "the mission will fail in its task if the regime continues to violate its commitments and obstructs the work of the mission."

The truce and the observer mission are part of special envoy Kofi Annan's plan for ending 13 months of violence and launching talks between Assad and those trying to oust him. Syria's opposition and its Western supporters suspect Assad is largely paying lip service to the cease-fire since full compliance could quickly sweep him from power.

So far, the regime has ignored such provisions and instead continued attacking opposition strongholds, though on a smaller scale than before the truce deadline.

Rice, in the toughest speech on Syria yet, warned that if Assad doesn't make good on all commitments or obstructs the monitors' work, the United States would pursue other "measures," which in diplomatic talk often means sanctions.