Saudi Arabia, a key backer of Syria's rebels, has welcomed a ceasefire brokered by Moscow and Washington, but stressed President Bashar al-Assad's regime must abide by it.

A foreign ministry source quoted by the official Saudi Press Agency said that the truce which began at sundown on Monday "will contribute to relieving the humanitarian suffering" in Syria, and Riyadh is following developments "with interest."

"The kingdom stresses the importance of Bashar al-Assad's regime and his allies abiding by this truce," the source said late Tuesday, expressing hope that it could ultimately lead to a peaceful transition of power.

UN envoy Staffan de Mistura said late Tuesday that there were reports of a "significant drop in violence" since the truce took effect.

But he said no aid had yet reached besieged civilians as foreseen by the truce deal because the UN was still seeking assurances for the safety of relief convoys.

Saudi Arabia has been a major backer of Islamist rebel groups in the civil war that erupted in Syria in 2011.

It was instrumental in the formation late last year of the High Negotiations Committee, an umbrella group uniting an array of rebel and political opposition factions.

Its rival Iran, the main regional backer of the Assad regime, has also welcomed the truce.