Syrians in the devastated city of Homs endured another day of heavy shelling Sunday, opposition activists said, a day after the U.N. Security Council voted to send up to 300 observers to monitor a shaky cease-fire. Elsewhere in the country, at least four people were killed by noon Sunday, the opposition Syrian Observatory for Human Rights said. The dead include two people killed by sniper fire in the Damascus countryside and a member of the Syrian security forces killed in Banias, the group said. The fresh violence in Homs erupted after a temporary halt in shelling Saturday, when two U.N. monitors were in the city. “”Today is the first day since two months … Homs (is) without shelling,”” one man told the monitors Saturday. “”When you come, shelling stops.”” He pleaded for the observers to stay in Homs, a bastion of anti-government sentiment that has faced months of deadly attacks by regime forces, opposition activists say. But snipers still targeted and killed five people in Homs on Saturday — part of the 40 people killed nationwide, said the Local Coordination Commitees of Syria, a network of opposition activists. The U.N. Security Council voted unanimously Saturday to authorize up to 300 unarmed military monitors to try and bring about compliance with a cease-fire imposed earlier this month. The cease-fire is part of a six-point peace plan laid out by U.N.-Arab League special envoy Kofi Annan, which was accepted by the Syrian government. The Annan plan calls on both the government and the opposition to end the violence, allow access to humanitarian groups, release detainees and begin a political dialogue. But reports of daily violence suggest the cease-fire is unraveling. Susan Rice, the U.S. ambassador to the United Nations, warned that while Washington supports expanding the U.N. monitoring mission, it may “”not agree to renew”” the 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.”” 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. Syria has consistently blamed “”armed terrorist groups”” for the violence wracking the country. 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 an agreement Thursday on a protocol for the advance monitoring team and other observers. Reports of bloodshed dropped in the days immediately after the cease-fire deadline, but accounts of terror and violence have since ratcheted up, with scores of people killed last week, activists said. CNN cannot independently verify reports of violence and deaths, as the government has severely restricted access by international media. 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.