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 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 an eight-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. Two observers stayed behind in Homs to keep monitoring the city, after the rest of the team left Saturday evening.

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's 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 U.S. 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.

Syria's U.N. ambassador, Bashar Ja'afari, told the Security Council that Syria informed Annan on Saturday that it has withdrawn troops and heavy weapons from urban centers, but he did not make clear when it occurred. Opposition activists said that in some areas, such as Homs, armored vehicles were moved off the streets Saturday, but remained near populated areas.

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 language usually means sanctions.

"Let there be no doubt. We, our allies and others in this body are planning and preparing for those actions that will be required of all of us if the Assad regime persists in the slaughter of the Syrian people," she said, adding the U.S. will not wait 90 days to take these measures if Syria keeps flouting its obligations.

Despite the violations, the international community sees Annan's plan as the only way forward.

Russia and China have shielded their ally Syria against Security Council condemnation, Western powers oppose military intervention and Gulf country have failed to keep promises of funding rebels.

On Saturday, five observers toured rebel-held areas in Homs, a center of the uprising that has been battered by tank and mortar shells for weeks. Previously, the Syrian regime, citing security issues, had turned down a request by the observers to visit the city.

"We did not hear any shelling today," said a Homs activist, who only identified himself as Abul-Joud, for fear of repercussions.

At one point, gunfire went off in the distance while the observers were in the Bayada neighborhood, accompanied by residents. The group ran into a house to take cover, according to Abul-Joud, who said he was walking with the observers. He said it did not appear the shots were aimed in the direction of the monitors.

In the Jouret el-Shayah neighborhood of Homs, observers were quickly thronged by residents who chanted, "The people want military intervention," according to video broadcast on the Al-Jazeera satellite TV station.

The observers walked silently through the streets in amateur video from the same neighborhood, posted online Saturday. A man in military uniform, apparently a rebel, pointed to the destruction, telling the team that "it's all destroyed buildings." Dozens of residents chanted, "The people want to execute the president," and "Freedom forever, against your will, Assad."

A spokesman for the observers, Neeraj Singh, said two observers stayed on after Saturday's tour "and have now been deployed in Homs as of this evening." Their presence could discourage a resumption of regime shelling.

The advance team is to increase to 30 monitors next week, before the larger contingent arrives.

Under a preliminary agreement between the U.N. and the Syrian government, the enlarged mission will be able to walk and drive freely through the country. However, Syria has so far not agreed to a U.N. demand that observers use their own planes and helicopters, seen as a key to the mission's success because it could reduce friction on the ground.

The challenges facing monitors became apparent this week when the advance team visited several hotspots. Large crowds of regime opponents surrounded the monitors, and government troops opened fire to disperse the protesters, in one instance while the observers were still present.

Reflecting concern about inadvertently setting off violence, the team decided not to go out on Friday, the main day for anti-government protests. The team leader, Col. Ahmed Himiche, said he and his men did not want to be "tools for escalation."

Hilal Khashan, an analyst at the American University of Beirut, said he believes the regime will try to sabotage the larger mission because it could pose a threat.

The presence of observers "attracts large crowds and anti-regime demonstrations," said Khashan. "If the observers spread throughout Syria, the extent of the protests will increase dramatically."

Syria denies it is facing a popular uprising, claiming it is being targeted by a foreign-led conspiracy of criminals and terrorists. Ja'afari, the Syrian U.N. envoy, reiterated Saturday that the regime reserves the right to respond to "armed terrorist groups."

Even Russia's envoy appeared to be skeptical about the regime's latest claims that it has pulled troops and tanks from Syria's cities.

"If indeed this is the case, this is a very important step in implementation of the Kofi Annan plan," said Russia's U.N. ambassador, Vitaly Churkin.


Associated Press writer Edith M. Lederer at the United Nations and Bassem Mroue in Beirut contributed reporting.