The U.N. and Arab League envoy for Syria, Kofi Annan, said Tuesday the next 48 hours must "bring visible signs of immediate and indisputable change in the military posture of the government forces" in Syria.
In a letter to the U.N. Security Council, Mr. Annan disputed Damascus' assertion that it is withdrawing its military from several cities, saying the Syrian army is pulling back from some areas but moving to others not previously targeted.
Watch a related report by Henry Ridgwell
The letter also urged the opposition to keep its commitment to stop fighting under the six-point peace plan, which demands an end to all hostilities by 6 a.m., Damascus time, on Thursday.
But despite continued violence, Mr. Annan said the peace plan is "still on the table."
He said: "So let me again appeal to the Syrian government and the Syrian parties to cease violence in accordance with the plan. And I believe there should be no preconditions for stopping violence. There is something we need to do for the people and for the country concerned."
Members of the Security Council, meanwhile, expressed deep concern about the level of commitment to a cease-fire the Syrian government has demonstrated so far.
U.S. Ambassador Susan Rice, who holds the council presidency this month, said that if Syria fails to fulfill its obligations, then the international community and the Security Council will have to decide whether to remain unified and take the next step. She said that would be to increase pressure on the Assad regime through collective action.
On the ground, however, violence continued. Syrian rights groups said at least 31 people, mostly civilians, were killed in the government shelling Tuesday - mainly in the central Hama region and the northwestern town of Mareh.
"Annan's peace plan all but dead"
Middle East expert Salman Shaikh says Mr. Annan's peace plan has virtually collapsed.
"His six-point plan is dead on arrival. It is not working," he said. "What is not dead is diplomacy. But, I think we need to now look very seriously at what other parts diplomacy can take. In my view, it has to be a much more coercive effort which tries to pull the international community to do something in Syria."
Shaikh, the director of the Brookings Doha Center, says the Syrian president, Bashar al-Assad, sees force as his only option.
"I'm afraid what Assad has determined is that only a security approach will manage to keep the lid on the protests which have now spread through the length and breadth of his country," he said. "In fact, if he was to withdraw his forces in any credible fashion, he would be facing hundreds of thousands of people on the streets of Syria, something he cannot afford any longer."
Give peace a chance
Representatives of Syria's main opposition alliance, the Syrian National Council, said Tuesday at least 1,000 people have been killed by government forces in the last eight days.
SNC spokeswoman Basma Kodmani said there were no indications that President Assad was honoring terms of the cease-fire, and stressed the opposition group could not accept a partial withdrawal of government forces.
Kodmani added that despite the killings, the opposition alliance is still willing to give peace a chance, but that it will explore other options if diplomatic efforts to end the crisis fail.
Deep skepticism among Syrian refugees
Mr. Annan's efforts to end the ongoing bloodshed in Syria have been met with deep skepticism by Syrian refugees, many of whom are calling for direct intervention by the international community against the government's crackdown.
Mr. Annan visited the Yayladagi refugee camp in Turkey Tuesday.
"I have had the chance to meet men and women who have been uprooted from their homes and are now here," he said. "I hope it is not going to be for long. But to hear their stories, to hear how they came across, how they were shot at, some with their children, was heart-wrenching."
Mr. Annan praised Turkish authorities for the facilities they have provided, saying the camps were better than most he has visited.
But observers say Mr. Annan is aware the Turkish government is increasingly concerned about the deteriorating situation in Syria and the fear the country could be faced with a humanitarian disaster.
Ankara has warned it could intervene in Syria if the numbers of refugees markedly increased. Due to the situation, Turkish Prime Minster Recep Tayyip Erdogan cut short a visit to China and is to visit Saudi Arabia on Friday to discuss the issue.
Also Tuesday, U.S. Senators Joseph Lieberman and John McCain visited the camp and issued a statement accusing the Syrian president of having no interest in complying with the Annan plan. The senators said Mr. Assad is buying time to intensify his crackdown.
"Make no mistake: the situation in Syria is an armed conflict. This is a war," the statement said.
Dorian Jones in Istanbul and Lisa Schlein in Geneva contributed to this report.