The UN's Syria envoy on Tuesday applauded a fall in violence through the first 24 hours of a fragile ceasefire but said security concerns meant aid convoys had not yet deployed.

Staffan de Mistura told reporters that the powers which brokered the truce -- the United States and Russia -- would likely provide a full assessment of the pact in the coming days or hours.

But he said that information reaching his office indicated "a significant drop in violence."

He noted that isolated reports of conflict persisted, especially on Monday night.

Possible ceasefire violations included mortar shelling by some opposition groups along the Castello Road, the key supply route to the battleground city of Aleppo, de Mistura said.

But he added that by sunrise on Tuesday the broad picture was positive.

A key test of the ceasefire agreed last week is whether warring parties will allow desperately needed humanitarian supplies to reach Syria's embattled civilians.

De Mistura dismissed reports by Turkish state media that UN trucks on Tuesday crossed the border towards rebel held eastern Aleppo, which has not received aid since regime troops seized the Castello road in early July.

The envoy said he had "no information about any UN trucks moving at this stage," demanding "assurances that the drivers and the convoy will be unhindered and untouched."

Spokesman for the UN's humanitarian office (OCHA), Jens Laerke, separately confirmed that no UN convoys have deployed across a border or within Syria since the ceasefire call.

The UN has said it is ready to send major stocks of life-saving aid to Syria's besieged and conflict-wracked areas.

Speaking earlier in Geneva, Laerke told reporters that Syrian President Bashar al-Assad's government had granted permission for aid to reach more than one million civilians this month.

But various obstacles remain. Past convoys have been blocked by government or military officials objecting to the contents of the humanitarian cargo.

De Mistura also stressed that opposition leaders in rebel controlled areas needed to accept aid "with no conditions or preconditions" before material can be moved.

The Syrian truce marks the latest bid to end a conflict that has killed more than 300,000 people since March 2011.

De Mistura has voiced hope that a decline in violence and more relief to civilians could help pave the way for peace talks to resume.

He provided no specific timelines on restarting negotiations, but said the issue was likely to dominate when the UN General Assembly meets later this month.

Russian Deputy Foreign Minister Mikhail Bogdanov said Tuesday that de Mistura could invite government and opposition representatives to new peace talks "at the very beginning of October".