Diplomatic activity over Syria is expected to be stepped up on April 11, a day after Damascus ignored a deadline to withdraw troops and tanks from cities.
The international envoy on Syria Kofi Annan is due in Tehran for talks with senior Iranian officials on Syria.
Tehran is one of Syria's few remaining allies, along with Moscow and Beijing.
On the eve of his Tehran talks, Annan urged the UN Security Council to back efforts to get the troop withdrawal done by April 12, two days later than originally planned.
Annan's appeal comes in a letter to the 15-member Security Council.
Under Annan's plan, Syrian troops were due to withdraw from cities by early April 10. That was to be followed 48 hours later by a cease-fire between the government and opposition fighters.
In Washington, U.S. Secretary of State Hillary Clinton is due to hold talks on Syria with her Russian counterpart, Sergei Lavrov.
Clinton said "intensive talks" on Syria were under way in New York and other key capitals.
The U.S. ambassador to the UN, Susan Rice, said Security Council members had expressed concern over Damascus's commitment to the cease-fire.
Rice said the UN Security Council would soon face a "moment of truth" on whether to increase the pressure on the regime of Syrian President Bashar al-Assad.
"It's the U.S. view that we face very soon a moment of truth," Rice said, "where if in fact the government continues to fail to fulfill its obligations -- which it gives every indication it will continue to do -- the international community and this council, which have all backed up the Annan plan in its entirety, will have to determine whether they remain unified in support of Joint Special Envoy Annan, his plan -- the plan that we as a council have adopted, which puts the onus on the Syrian government to take the essential initial steps.
"And whether we are prepared if there is continued noncompliance to take the logical next step, which is to increase the pressure on the Assad regime through collective action," she added.
Annan said the onus was now on Damascus to use the next 48 hours to "bring signs of immediate and indisputable change in the military posture of the government forces throughout the country."
Annan also said the opposition should stop fighting in order to "give no excuse for the government to renege on its commitments."
On the ground inside Syria, there was no sign of a letup in the violence, with at least 30 reported killed on April 10.
Based on AP and Reuters reporting