(CNN) -- The U.N. Security Council on Saturday voted unanimously to authorize up to 300 unarmed military monitors to try and bring about compliance with a cease-fire imposed last week in violence-wracked Syria.
The resolution comes just hours after reports that a small group of U.N. cease-fire monitors had arrived in the besieged Syrian city of Homs, a flashpoint of a conflict that has gripped the country for more than a year.
Susan Rice, the U.S. ambassador to the United Nations, warned that while Washington supports the resolution, it may "not agree to renew" the U.N. monitoring mission at the end of 90 days.
"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 this mission has run its course," Rice said. "Our patience is exhausted."
Russian Ambassador Vitaly Churkin said his country also supports the resolution, reminding observers that the peace plan calls for both the government and opposition groups to cease hostilities.
"We're on the right track now," said Churkin.
Syrian security forces will exercise the "utmost degree of restraint," but also remain prepared to defend their national interests against terrorists, added Syrian U.N. Ambassador Bashar Jaafari.
Jaafari, who was speaking at the U.N. Security Council after the body approved the measure to send additional observers to his country, asked the monitors to remain objective in their assessments.
International observers, meanwhile, held talks with activists in Homs, a hotbed of dissent and the scene of protracted violence, an opposition group's Facebook page said. The observer team was expected to arrive in two hard-hit neighborhoods -- the old city and Khalidiya, the Homs Revolution page said.
The observers had met earlier with the Homs governor, the state-run Syrian Arab News Agency (SANA) reported Saturday.
They have visited several locations in Daraa province and the Damascus countryside, but the government had rejected their initial request to visit Homs this week because of security concerns.
Security forces have shelled opposition strongholds in Homs for weeks, activists said.
Activists also said the government is masking its military presence in the city to show adherence to a key element of the Annan peace plan -- the withdrawal of troops from population centers.
For instance, the Syrian military in Homs is hiding at least 17 tanks in two state-owned buildings, said the Local Coordination Committees of Syria, a network of opposition activists.
The resolution calls for Syrian regime's immediate implementation of a six-point peace plan laid out by U.N.-Arab League envoy Kofi Annan and calls for both the government and opposition to cease violence.
The Security Council previously approved the deployment of an advance team of 30 monitors meant to pave the way for the larger group of observers. The United Nations and Syria reached agreement Thursday on a protocol for the advance monitoring team and other observers.
Reports of bloodshed dropped in the days immediately after last week's cease-fire deadline, but accounts of terror and violence have ratcheted up since, with scores of people killed this week, activists said.
Forty people -- among them children, women and six members of the Free Syrian Army, made up primarily of Syrian military defectors -- were killed Saturday around Syria, the LCC reported.
Of those, 15 people were killed in Daraa -- including an entire family trying to flee to Jordan -- and at least five people in the besieged city of Homs were shot to death by snipers, according to the group. Syrian forces also arrested more than 60 people near the Jordanian border.
SANA blamed "armed terrorist groups" for ongoing violence, saying Saturday that such terrorists detonated a bomb that killed 10 law enforcement members in Daraa province, in the south. It also said terrorists sabotaged an oil pipeline in Deir Ezzor province, in the east.
CNN cannot independently verify reports of violence and deaths, as the government has severely restricted access by international media.
The international community wants an end to the bloodshed, but the Security Council has been split between Western countries demanding strong measures against Syrian President Bashar al-Assad, and Russia and China, the two countries on the 15-member Security Council that have quashed attempts to take tougher action against the Syrian regime.
Russia -- which, along with China, has blocked action against the Syrian regime -- had called for the quick approval of the measure to deploy more monitors. Russia also said a Syrian opposition delegation will visit Moscow next week.
China said it will send observers to join the U.N. monitoring mission, the state-run China Daily newspaper reported, citing a foreign ministry spokesman.
The Annan plan calls on both sides to end the violence, allow access to humanitarian groups, release detainees and begin a political dialogue.
On Saturday, SANA announced it had released 30 detainees whose "hands were not stained with the blood of the Syrians." The state-run agency also said hundreds of detainees had been released in November, December and January.
But Ban said this week there has also been no significant release of detainees and no substantive progress on providing humanitarian assistance, despite the Syrian regime's acceptance of the peace plan in March. He also said Syria has not lived up to its promise to withdraw troops from cities.
The Annan plan also calls for the right to demonstrate peacefully. Opposition groups said security forces in Aleppo and Daraa fired at demonstrators on Friday, the day of the week nationwide protests are held, activists said. At least 57 people were killed across Syria on Friday.
Syria has been engulfed in violence for 13 months, since the government started a fierce crackdown on peaceful protests against President Bashar al-Assad's regime. The president's family has ruled Syria for 42 years.
The United Nations estimates that at least 9,000 people have died since the protests began, while activist groups put the death toll at more than 11,000.
CNN's Joe Vaccarello, Saad Abedine, Ivan Watson and Amir Ahmed contributed to this report.