The Security Council has voted to increase the number of UN observers in Syria to 300 for three months.
A small UN team is currently in Syria to monitor a fragile ceasefire between government and rebel forces.
The UN resolution was unanimously approved by the 15-member council, as the monitors were allowed to visit the city of Homs for the first time.
The visit came amid a lull in fighting in the opposition stronghold, which has been under bombardment by the army.
Rebels said tanks had been temporarily hidden out of sight while the observers were in the city, and that shelling was likely to resume.
However amateur video posted on the internet shows gunfire breaking out during the visit and monitors being surrounded by Homs residents. It is not clear who is responsible for the firing.
The resolution was a compromise between a Russian and a European text. The main difference was over conditions required for sending the monitors.
The Russians didn't specify any criteria. Western states wanted the deployment contingent on government compliance with the ceasefire, especially its pledge to pull troops and heavy weapons back to barracks.
In the end, all agreed to let the Secretary General make the decision about when it would be safe enough to deploy the unarmed observers.
But Western diplomats made no secret of their concern about the fragile state of Kofi Annan's peace plan. The US ambassador Susan Rice warned that Washington wouldn't wait the full three months of the monitors' mandate to pursue measures against Damascus if it continued to violate its commitments.
But it's difficult to see what alternative the West has to the Annan plan, given that it doesn't support foreign military intervention, and would face Russian and Chinese vetoes for any UN sanctions.
A Homs activist calling himself Abo told the BBC that he and other activists had tried to protect the monitors.
"This is our first day of calm for months," he said. "The regime today didn't shell any area or open fire until the observers came here."
There is no word from the UN itself about the alleged incident.
The UN resolution was adopted following a debate about the conditions for deployment.
European states had said the unarmed observers should be sent only when Syria implemented its pledge to send troops and tanks back to barracks.
Russia - which is regarded as an ally of Damascus - simply emphasises the need to send more monitors to Syria quickly.
In the end, the resolution leaves it to UN Secretary General Ban Ki-moon to decide how and when they will be deployed.
Although overall violence has fallen since the truce was signed by the UN and Syria on Thursday, many violations have been reported by activists and journalists on the ground.
According to Reuters news agency, at least 23 people were killed on Friday, 10 of them in a roadside bomb targeting security forces and most of the others in army shelling on the city of Homs.
Footage emerged appearing to show UN observers under fire in Homs
The Damascus authorities say they are fighting armed terrorist groups and that the ceasefire allows them to respond to attacks.
The UN estimates that government forces have killed more than 9,000 people in the uprising. Syria says foreign-backed militants have killed more than 2,600 soldiers and police.
Russia's UN Ambassador Vitaly Churkin told the council after Saturday's vote that the resolution was "of fundamental importance" to push forward the six-point peace plan negotiated by international envoy Kofi Annan.
Annan's six-point peace plan
1. Syrian-led political process to address the aspirations and concerns of the Syrian people
2. UN-supervised cessation of armed violence in all its forms by all parties to protect civilians
3. All parties to ensure provision of humanitarian assistance to all areas affected by the fighting, and implement a daily two-hour humanitarian pause
4. Authorities to intensify the pace and scale of release of arbitrarily detained persons
5. Authorities to ensure freedom of movement throughout the country for journalists
6. Authorities to respect freedom of association and the right to demonstrate peacefully
Britain's envoy Sir Mark Lyall Grant said the expanded observer mission "represents the last opportunity to secure a solution to the crisis in Syria.
"US Secretary of State Hillary Clinton earlier in the week called for a global arms embargo and further sanctions if the government continued to break the ceasefire.
The international community has also been looking at ways of getting humanitarian aid to Syria, with diplomats meeting in Geneva on Friday to discuss the situation.
They agreed to a draft plan to provide $180m (£112m) for food, medicine and other supplies to about one million people inside Syria.
That comes on top of the aid that is being delivered to refugees who have fled to neighboring countries.