Russian ambassador to the United Nations Vitaly Churkin speaks during a Security Council meeting at the United Nations in New York April 21, 2012. The U.N. Security Council unanimously adopted a resolution on Saturday that authorizes an initial deployment of up to 300 unarmed military observers to Syria for three months to monitor a fragile week-old ceasefire in a 13-month old conflict. (ALLISON JOYCE/REUTERS)

The U.N. Security Council voted Saturday to establish a full-fledged U.N. mission, with up to 300 unarmed military observers and an unspecified number of civilian specialists, to monitor a shaky cease-fire between the Syrian government and armed opposition forces.

The newly minted U.N. Supervision Mission in Syria is set to reinforce a small advance team that began testing the nine-day-old cease-fire this week with visits to a handful of Syrian towns, including a trip Saturday to the city of Homs, the scene of a military crackdown in recent months.

The agreement marks a public show of unity among the United Nations’s fractious big powers in support of U.N.-Arab League envoy Kofi Annan’s six-point plan for ending 13 months of deadly upheaval and clearing the way for a political settlement between President Bashar al-Assad’s government and a diverse array of armed and civilian opponents. The Security Council resolution authorizes the new mission for an initial 90 days but does not include a timetable for its deployment, leaving that decision to U.N. Secretary General Ban Ki-moon.

After the vote, Susan E. Rice, the U.S. ambassador to the United Nations, said that although the Obama administration supports the move, there should be “no illusions” that a small mission of U.N. observers will necessarily be capable of halting the Syrian crackdown and that the United States is prepared to pull the plug on it after 90 days if Syria does not comply with Annan’s peace plan.

“We are sober about the risks, all the more given the Assad regime’s long record of broken promise, deceit and disregard for the most basic human standards,” she said. “Let there be no doubt that 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.”

Syria’s U.N. ambassador, Bashar al-Jaafari, said that the United States and its European and Arab allies were seeking to undermine the U.N.-brokered peace process and that their public expressions of doubt about its prospects for success were emboldening the Syrian opposition to continue fighting. The opposition’s foreign supporters, Jaafari told the council, must “cease to fund, arm and train the armed groups and desist from encouraging them to continue their terrorist actions.”

Meanwhile, members of the small advance team of observers visited Homs, where residents reported that the military had halted its artillery bombardment for the first time in more than a week. Cellphone coverage also returned for the first time in months, but amateur video footage of the observers driving and walking through Homs neighborhoods indicated gunfire near one patrol.

According to Ban, the Syrian government, which is responsible for the observers’ safety, had previously prevented the team from visiting Homs, citing security reasons. In one video shot Saturday by activists in the suburb of Khalidiyeh, which has seen fierce fighting in recent months and been heavily shelled by government forces, a weeping woman bangs on the window of the U.N.-marked vehicle, trying to pass the observers a letter.

In another, as two monitors walk in the street with chanting residents waving the flag of the uprising, the sound of gunfire can be heard and the demonstrators hustle the observers to the side of the road, apparently to protect them. No injuries were immediately reported.

Homs has been a center of rebellion against the government for months, with peaceful protesters and armed rebels both present. Thousands are believed to have died in fighting between security forces and the heavily outgunned opposition, and some neighborhoods, particularly the hard-hit Baba Amr area, have been badly damaged, with many buildings ruined.

Activists and diplomats say that in contravention of the cease-fire agreement, the government has kept a military presence in the city, while the government says armed groups are still fighting there. Activists said Saturday that some tanks had been hidden in trenches ahead of the U.N. visit.

After protests Friday in which dozens of people — including civilians, security forces and armed opponents of Assad — were reportedly killed, the country was relatively quiet Saturday, according to the Britain-based Syrian Observatory for Human Rights. Syrian state media reported, however, that an “armed terrorist group” blew up an oil pipeline near Deir al-Zour, in the east, and that four policemen were abducted in the restive northern province of Idlib.

The adoption of the supervision mission resolution Saturday followed a day of intensive negotiations in which Britain and France, on the one hand, and Russia, on the other, tabled competing resolutions that reflected the sharp differences over the U.N. role in the crisis.

Britain and France, supported by the United States, favored the inclusion of a provision threatening Syria with sanctions if it does not withdraw its troops and military equipment to barracks.

Russia, however, offered a milder text that contained no threat of sanctions and sought to spread blame for the crisis. In the end, the council began negotiations on the basis of the Russian draft, striking a compromise that includes an expression of “profound regret” at the deaths of thousands of Syrian civilians and condemns “widespread violations of human rights by the Syrian authorities, as well as any human rights abuses by armed groups.”

The resolution reiterates U.N. calls on Syria to guarantee freedom of movement for the monitors, and it prods the government to swiftly conclude an agreement allowing the United Nations to use planes and helicopters in Syria to quickly transport monitors to trouble spots.

The resolution increases pressure on Syria to rapidly implement all six elements of Annan’s plan, including the release of political prisoners, guarantees of freedom of movement for international aid workers and journalists and the right to hold peaceful protests. But it goes beyond it in the call for Syria to withdraw its forces and equipment to barracks. Annan’s plan only requires Syria to begin that process.

On Thursday, Ban offered a mixed account of the success of the cease-fire, telling the Security Council in a report that “it remains a challenge to assess accurately unconfirmed and conflicting reports of developments in Syria.”

Ban wrote that “levels of violence dropped markedly” in Syria after April 12, when the cease-fire went into effect. “However,” he said, “the Syrian Government has yet to fully implement its initial obligations regarding the actions and deployments of its troops and heavy weapons, or to return them to barracks. Violent incidents and reports of casualties have escalated again in recent days, with reports of shelling of civilian areas and abuses by government forces. The Government reports violent actions by armed groups. The cessation of armed violence in all its forms is therefore clearly incomplete.”

After the vote, Russia’s U.N. ambassador, Vitaly I. Churkin, praised the outcome, saying it “enshrined the council’s consensus” behind the Annan plan. But he faulted the Syrian opposition’s backers for their pessimism, saying they bore a responsibility to “uphold the resolutions of the Security Council and not undermine its activity.” He urged them to use their influence to encourage the opposition to meet their own obligations under the cease-fire agreement.

Rice and other Western diplomats voiced deep skepticism about Syria’s commitment to the Annan plan, saying Damascus has continued shelling neighborhoods in Homs and Idlib despite the cease-fire.

“Let me be plain,” Rice said. “If there is not a sustained cessation of violence, full freedom of movement for U.N. personnel and rapid meaningful progress on all other aspects of the six-point plan, then we must all conclude that this mission has run its course.”

Fordham reported from Beirut.