Ahmad Fawzi acknowledged continuing violence in the country since the cease-fire took effect, the United Nations said in a statement. Annan, former U.N. secretary-general, represents both that organization and the Arab League.
But Annan says the government of President Bashar Assad has removed heavy artillery from some areas.
"The Annan peace plan is on track, and a crisis that has going on for over a year is not going to be resolved in a day or a week. Sadly, time is a luxury that we don't have, but realistically it's going to take a little more time to pull all the strings together," Fawzi said. "There are days when things are progressing in a satisfactory manner and there are day where we feel that things are so difficult that we need a lot of patience and perseverance to see the day through. We are horrified by the extent of violence that we see on the ground."
Syrian activists said at least 37 people were killed Friday by government forces firing on protesters or after being arrested, CNN reported. Three in Idlib who were detained earlier this week appeared to have been tortured, rebels said.
A spokesman for U.S. Secretary of State Hillary Clinton was more pessimistic than Annan.
"If the regime's intransigence continues, the international community is going to have to admit defeat and work to address the serious threat to peace and stability being perpetrated by the Assad regime," White House spokesman Jay Carney told reporters Thursday, after Syrian security forces stormed an anti-government protest at Aleppo University, killing at least four students -- including one heaved out a fifth-floor window -- and forcing the state-run school to close, activists and opposition groups said.
If the international community admits the peace plan brokered by U.N.-Arab League envoy Kofi Annan last month has failed, Washington will work with the Security Council and countries and groups outside the Security Council to find a "political transition," Carney said, adding such a transition was "urgently needed."
"It is clear, and we will not deny, that the plan has not been succeeding thus far and that the regime has made no effort to take any of the steps required under the Annan plan, including moving toward the implementation of a full cease-fire," he said.
The Obama administration hopes the Annan plan succeeds, Carney added. But "based on the evidence," the administration remains "highly skeptical of Assad's willingness to meet the conditions of that plan because he has so clearly failed to meet them thus far."
Aleppo University -- Syria's second-largest after the University of Damascus, with 61,000 undergraduate students, more than 1,500 graduate students and some 2,400 faculty members -- said it would suspend classes until final exams May 13.
It said it closed the student dormitories Syrian forces stormed overnight Wednesday into Thursday, firing tear gas and then live ammunition, and arresting scores of people, after some 1,500 students protested against Assad's regime.