The United Nations Secretary General Ban Ki-moon has said the situation in Syria remains "highly precarious".
The UN has reached a preliminary deal with the Syrian government on rules governing the deployment of observers to monitor an agreed ceasefire.
Mr Ban said he has seen "troubling" evidence of ongoing violence.
US Secretary of State Hillary Clinton called for a global arms embargo and further sanctions if the government continues to break the ceasefire.
The BBC's Ian Pannell in northern Syria reports seeing numerous breaches of the ceasefire over the last week.
Our correspondent says he saw a helicopter gunship firing on villages in the Jabel al-Zawiya area, killing at least two people.
'Path to civil war'
Foreign ministers from the Friends of Syria coalition, meeting in Paris on Thursday evening, agreed that the ceasefire and Kofi Annan's six-point peace plan were the "last hope" of avoiding civil war in Syria.
Mrs Clinton called for a Security Council resolution to impose UN sanctions on Syria if President Bashar al-Assad blocked the monitors' mission.
The Friends of Syria group includes Western and Arab nations, but not Russia or China, who have blocked previous attempts to introduce UN sanctions.
Mrs Clinton said she had met her Russian counterpart, Sergei Lavrov, earlier in the day, and he recognised that the situation in Syria was "deteriorating".
The French Foreign Minister Alain Juppe said that if the UN-backed peace plan failed, Syria would be "on the path to civil war".
Mr Juppe warned the situation in Syria could spill out to the wider region, and said "several hundred" international monitors were needed.
Mr Ban stressed Syria's responsibility to provide "unfettered access" and use of helicopters to the monitors, the first of whom arrived this week.
The agreement reached with Damascus does not include access to aircraft, according to the Reuters news agency.
It is also not clear how freely observers will be permitted to move around under the terms agreed with the Syrian authorities.
Mr Ban says he wants the UN observer mission to increased to 300, to be deployed over three months.
Some correspondents say the Security Council, which is not due to make a decision on the mission until next week, may be afraid to put unarmed observers on the ground if the situation continues to deteriorate.
The UN chief said such a deployment would not be without risk, but UN supervision, with a clear mandate and the right capacity, would contribute to improving the situation on the ground.
He also called on Syria to allow UN agencies to mount a "major humanitarian field operation", which he said was needed to help the estimated one million people affected by the uprising.
The Syrian Observatory of Human Rights, an activist group, said fresh fighting on Thursday killed at least three people across the country.
There has also been further government shelling of parts of the third biggest city, Homs, the BBC's Jim Muir reports from neighbouring Lebanon.
Deputy Foreign Minister Faisal al-Mekdad and a member of an advance team of observers, which arrived in the country earlier this week, signed the preliminary accord in Damascus, the Syrian foreign ministry said.
"This agreement comes within the framework of Syrian efforts aimed at making the Annan plan succeed and to facilitate the UN observer mission while respecting Syria's sovereignty," a statement said.
The office of Mr Annan confirmed an agreement had been signed, and said it was also having discussions with representatives of Syrian opposition groups.
"This agreement outlines the functions of the observers as they fulfil their mandate in Syria and the tasks and responsibilities of the Syrian government in this regard," a statement said.
In a report to the UN, Mr Ban said the Syrian government had not complied with the terms of the peace deal, but that there remained an "opportunity for progress".
While violence had fallen at the time the truce came into effect on 12 April, it had escalated again in recent days, he said. Mr Ban said it was critical that the government kept its pledge to withdraw troops from populated areas.
He also noted that there had been "no significant release of detainees", and "no substantive progress" in negotiations on humanitarian access, in reference to other aspects of Mr Annan's six-point plan.
He said observers had been prevented from going to the city of Homs after the government cited "security concerns". But observers were allowed to visit the town of Deraa freely.
Our correspondent says that while the trip to Deraa took place without incident, observers found themselves mobbed by protesters in the Damascus suburb of Arbeen.
Gunfire broke out and the observers had to get out, he says. Several demonstrators were injured, according to activists.
In his report, Mr Ban said: "The situation in Arbeen became tense when a crowd that was part of an opposition demonstration forced United Nations vehicles to a checkpoint."
"Subsequently, the crowd was dispersed by firing projectiles. Those responsible for the firing could not be ascertained by the United Nations military observers."
Ahead of the Friends of Syria meeting in Paris, President Sarkozy of France accused the regime of trying to wipe out Homs altogether, and called for the imposition of humanitarian "corridors" in Syria.
Also on Thursday, Britain announced that it was contributing £4m ($6.4) to a UN fund for humanitarian needs caused by the conflict, on top of more than £4m it has already disbursed.