The truce between President Bashar al-Assad's regime and non-jihadist rebels, brokered by Russia and the United States, has defied expectations and led to the first significant decline in violence in Syria's nearly five-year civil war.
The United Nations is hoping it can now restart talks on a political transition that collapsed last month in Geneva.
The opposition, represented by the Riyadh-based High Negotiations Committee, had held off on committing to the talks but on Monday said the 10-day-old "cessation of hostilities" was making a difference.
"We have noticed a sharp decline in ceasefire violations in recent days and progress in the humanitarian file," particularly with regards to aid deliveries to besieged towns, Agha said.
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The ceasefire agreed on February 27 is part of the biggest diplomatic effort yet to resolve Syria's conflict, which has killed more than 270,000 and forced millions of people from their homes.
Russia and the United States are on opposing sides of the conflict -- Moscow backs Assad and Washington supports the opposition -- but the two powers have made a concerted push for the ceasefire and further peace efforts to succeed.
The Syrian Observatory for Human Rights, a Britain-based monitor, said Sunday had been the "calmest day" in Syria since the ceasefire began.
Observatory chief Rami Abdel Rahman told AFP that the average number of civilian deaths a day had fallen by 90 percent since the ceasefire came into force, with an 80 percent decline among soldiers and rebel forces.
Moscow, which has provided a daily account of ceasefire violations, said today that the truce was still "in general" holding apart from unspecified "isolated provocations and shelling".
Russia launched an air campaign in September it says is targeting "terrorists" but has been accused of hitting non-jihadist rebels in support of Assad's forces.