Western nations have given a sceptical welcome to President Bashar al-Assad's acceptance of a UN-Arab League peace plan for Syria as the UN said the death toll for the uprising has passed 9,000.
"Violence on the ground has continued unabated," Robert Serry, a UN Middle East peace envoy, told a Security Council meeting.
Mr Serry added: "Credible estimates put the probable death toll since the beginning of the uprising one year ago to more than 9,000.
"It is urgent to stop the fighting and prevent a further violent escalation of the conflict."
The Security Council discussed the Middle East after UN-Arab League envoy Kofi Annan announced that Mr Assad has accepted his six-point plan.
The plan includes a commitment to halt violence and allow a two-hour humanitarian pause to fighting each day.
Activists said there had been new deaths in Syria despite the response to Mr Annan and concerns were raised in Security Council consultations over reports that Syrian forces had crossed into Lebanon to an area where Syrian opposition members have taken refuge, diplomats said.
Reports from Syria cannot be independently verified as state authorities have barred international journalists and rights groups.
Mr Annan, who will brief the Security Council on Monday, called on the Syrian government to "put its commitments into immediate effect".
Western powers said Syria's actions now will be a test of its attitude to international calls to halt killings.
US State Department spokeswoman Victoria Nuland called Mr Annan's announcement "an important step".
Ms Nuland said: "But as with all things with the Assad regime, the proof will be in the actual action that he takes.
"We will be looking for him to take immediate action to begin implementing Annan's proposals, starting with silencing his guns and allowing humanitarian aid to go in."
Britain's Foreign Secretary William Hague also said Mr Assad's acceptance of the plan could be "a significant first step" towards ending the deadly crackdown on the opposition.
"But only if it is genuinely and seriously meant," Mr Hague said in London. "This has not been the case with previous commitments the regime has made.
"The key will be concrete implementation that brings a cessation of all hostilities and leads to a genuine political transition.
"We will continue to judge the Syrian regime by its practical actions not by its often empty words."
Germany's UN ambassador Peter Wittig also urged caution.
"Syria has in the past a history of credibility gaps," Mr Wittig told reporters.
The Security Council passed a statement last week that called on the Syrian regime to halt violence and the use of heavy weapons.
"That we have not seen on the ground," said Mr Wittig, who added that Syria's action now "will be the litmus test whether they comply with that".