An advance team of observers was touring Syrian hot spots on Sunday, laying the ground for a larger mission unanimously backed by the UN to monitor a shaky ceasefire aimed at ending bloodshed.
The UN Security Council voted Saturday to send an extra 300 unarmed observers to Syria, but the United States warned it may veto a new mandate for the mission.
Two members of the advance team remained in the restive city of Homs on Sunday, a mission spokesman told AFP, a day after they made their first visit to the central protest hub since being deployed in Syria a week ago.
On Saturday, their visit to Homs included a stop in Baba Amr, a rebel stronghold battered by a month-long army bombardment that killed hundreds, according to monitors, before it was retaken on March 1.
Video uploaded to YouTube showed four of them meeting with activists who begged them to stay in the central city. Its authenticity could not immediately be verified.
"Today is the first day since two months, exactly since 5 February... in Homs without shelling... without killing, without fire," one unidentified activist said in the footage.
"Because of that, we want you to stay. Please stay. This is what we want. When you come, shelling stops. When you come, killing stops," he told the observers, who wore blue helmets and bullet-proof vests marked "UN".
The visit came as the an activist in Homs said the situation in the city was calm.
In Banias, meanwhile, the Syrian Observatory for Human Rights said an overnight ambush on a patrol killed one security forces member and wounded three others, the first such incident in the coastal city for nearly a year.
Only days after being deployed, the team of seven observers in Syria acknowledged they faced a tough task to firm up the ceasefire agreed to last week by embattled President Bashar al-Assad.
A spike in violence had already forced the Arab League to end its own Syrian monitoring mission in late January, barely a month after it was launched.
Under UN Resolution 2043, adopted unanimously on Saturday, 300 military observers will be sent to Syria for an initial period of 90 days if UN Secretary General Ban Ki-moon determines it is safe for them to go.
The UN says well over 9,000 Syrians have been killed since democracy protests erupted in March 2011, inspired by uprisings that toppled autocratic regimes in Tunisia, Egypt and a revolt that ousted Libya's Moamer Kadhafi.
Monitors put the figure at more than 11,000, including at least 200 people killed in sporadic violence which has persisted since a UN-backed ceasefire went into effect on April 12.
Russia's UN envoy Vitaly Churkin, whose country played a leading role in drawing up the resolution, told the Security Council the measure was of "fundamental importance to push forward the process of the peaceful settlement in Syria."
The United States, however, warned it may prevent the renewal of the mission after the first three months, while urging greater international pressure on Assad's government.
Susan Rice, the US ambassador to the United Nations, told AFP that Ban must make a "careful judgment" about conditions in Syria before sending the larger contingent of unarmed monitors there.
"Our patience is exhausted. No one should assume that the United States will agree to renew this mission after 90 days," Rice told the Security Council after it authorized the full mission.
France's UN Ambassador Gerard Araud backed the United States by saying there was "no automatic renewal" of the observer mission, adding that it should only be renewed "if we think it is useful."
Churkin, the Russian envoy, called Rice's remarks "unhelpful."
"Making negative predictions sometimes looks like a prophecy which some people want to be borne out," he told reporters.
The opposition Syrian National Council and the rebel Free Syrian Army hailed the UN Security Council vote, saying it responds to the Syrian people's demands.
"Undoubtedly, the sending of new monitors is a demand of the Syrian people and of the revolutionaries who protest every day," SNC spokesman George Sabra told AFP, welcoming the unanimous council decision.
"But I do not think that 300 monitors will be enough for a country like Syria, where the revolution is affecting every town and village," he added, calling for the mission to be enlarged to make it "more effective.
The rebel army said it remained "committed to the ceasefire" but spokesman Colonel Kassem Saadeddine stressed that "protection of civilians is the responsibility of the international community."
He accused government forces of failing to respect the truce, saying "there are daily bombardments... and tanks are still on the streets" in violation of the peace plan.