The UN's Syria envoy on Tuesday applauded a "significant drop" in violence through the first 24 hours of a fragile ceasefire but said security concerns meant aid convoys stayed on hold.

The truce brokered by Russia and the United States began at sundown on Monday, in the latest bid to end a conflict that has killed more than 300,000 people since March 2011.

The agreement aims to bring an end to fighting between President Bashar al-Assad's loyalists and a wide range of rebels but excludes jihadist forces like the Islamic State (IS) group.

UN envoy Staffan de Mistura said reports reaching his office indicated "a significant drop in violence", in a rare respite in Syria's devastating conflict.

De Mistura noted isolated reports of conflict persisted, especially on Monday night, but that by sunrise on Tuesday the broad picture was positive.

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".

In second city Aleppo, AFP correspondents in both the rebel-held east and the government-held west reported nearly 24 hours had passed without air strikes or rocket fire.

AFP correspondents in government-held Damascus and its rebel-controlled suburbs reported quiet too, with residents taking advantage of the lull in violence to mark the Eid al-Adha Muslim holiday.

The Britain-based Syrian Observatory for Human Rights reported minor violations by both government and rebel forces in different parts of the country, but no deaths.

However, the truce did not prevent heavy fighting between regime forces and the jihadist Jund al-Aqsa faction in Hama province of central Syria, the monitoring group said.

Damascus accused the opposition of a series of violations, with its ally Moscow saying Syrian government forces were fully respecting the truce but that rebels had violated it 23 times.

"Syrian government troops have completely stopped firing" except in jihadist-held areas, but "the same cannot be said for armed units of the moderate opposition controlled by the US", said Viktor Poznikhir, a senior Russian military officer.

- 'We could sleep' -

The lull in violence was a rare respite for residents of the war-ravaged country, where more than half the population has been displaced and hundreds of thousands live under siege.

"We usually stay up all night with the airplanes, but thank God last night we could all sleep," said activist Hassan Abu Nuh in opposition-held Talbisseh in central Syria.

It was also quiet in the largely rebel-held northwestern province of Idlib, where air strikes killed 13 people only hours before the truce.

In rebel-held eastern Aleppo, children laughed as they ran through the streets, some playing football under a destroyed bridge.

"The truce is good, but it's not enough. We want food to come in," said resident Abu Jamil.

In the government-held west of the city, Habib Badr was enjoying the silence.

"My house is near the Razi hospital and I'm used to hearing ambulance sirens every two or three hours. I haven't heard anything this morning," he said.

The deal is the latest in a succession of attempts to end the fighting in Syria.

It calls for the truce to be renewed every 48 hours, and immediate humanitarian aid access, particularly to civilians living under siege.

By Tuesday afternoon, there were no signs aid had begun moving into those areas, including eastern Aleppo which is surrounded by government troops.

Damascus warned, meanwhile, that all aid going to Aleppo, particularly assistance sent by Turkey, must be coordinated with it and the United Nations.

Russia said its troops had been deployed on the key Castello Road running from Turkey into Aleppo that is to become a demilitarised zone under the deal.

- Opposition sceptical -

If the ceasefire holds for a week, Moscow and Washington will then begin an unprecedented joint campaign to target jihadists, including IS and former Al-Qaeda affiliate Fatah al-Sham Front.

Russia said De Mistura could invite government and opposition representatives to new peace talks "at the very beginning of October".

But there is still deep scepticism about whether the truce will last, with the opposition yet to officially sign on.

A crucial part of the deal calls on non-jihadist rebels to break ranks with Fateh al-Sham ahead of joint US-Russian operations against the group.

But many Islamist rebel groups cooperate closely with Fateh al-Sham, and the biggest of them -- the powerful Ahrar al-Sham group -- has criticised the terms of the Russian-US deal.

Damascus and its allies have backed the truce, but on Monday Assad said his regime remained "determined to recover every area from the terrorists".

The Pentagon said Tuesday that its recent strikes on IS forces, before the ceasefire began, "may have resulted in civilian casualties."

The incidents took place September 7, 10 and 12, according to a statement from CENTCOM. It did not give a number of dead or injured.

US accuses Syria of blocking aid to besieged civilians
Washington (AFP) Sept 15, 2016 - The United States accused Syria on Thursday of blocking aid to besieged cities and warned it will not boost military cooperation with Russia unless Damascus honors a truce agreement.

President Barack Obama's spokesman Josh Earnest said Russia is responsible for ensuring that its ally Syrian leader Bashar al-Assad allows United Nations convoys to enter towns like Aleppo.

"Right now, the trucks that could bring them life-saving assistance are idling on the wrong side of the border," Earnest told reporters.

"And that's the direct responsibility of the Assad regime and their benefactors in Moscow."

Separately, Pentagon spokesman Peter Cook warned that unless Russia is able to convince Assad both to halt attacks on rebel groups and allow humanitarian access, military cooperation will not happen.

Last week, US Secretary of State John Kerry and Russia's Foreign Minister Sergei Lavrov agreed to pressure Syria's warring parties to stop fighting and allow in aid.

Russia was to convince Assad to accept the deal, while America was responsible for dealing with a coalition of opposition rebel groups.

Once Washington and Moscow can jointly agree that a ceasefire has held for seven consecutive days, they are to open a joint cell to coordinate US-Russian strikes on jihadist groups.

The truce came into effect at nightfall on Monday, but State Department spokesman Mark Toner refused to be drawn on how many days of ceasefire have elapsed or when cooperation could begin.

"Certainly we're not there yet," he said. "We're not there and we're not claiming to be there."

- Skeptical Pentagon? -

There have been reports that the US Defense Department is more skeptical of the deal with Russia than is Kerry's State Department, and Cook warned the terms are not yet met.

"We've said that there would need to be the sustained period with the reduction in violence, with the terms that would need to be complied with," Cook said.

"And we haven't seen that yet," he said, stressing the need for aid to move freely. "So there will be no military coordination until those terms have been met."

Toner said there had been breaches of the ceasefire by both sides in the conflict and, more worryingly, Assad's forces have not allowed UN aid convoys through.

Kerry and Lavrov did not speak Thursday, he added, but "but it continues to be our assessment that this is worth pursuing. It's worth continuing."

Washington has not released the full text of its agreement with Russia to the frustration of observers and now allies like France, which has complained about the secrecy.

Toner insisted that US allies would be briefed in detail on the deal during meetings on the sidelines of the United Nations General Assembly in New York next week.

But the spokesman said the text of the agreement deals with "sensitive issues" and could be "misused or misinterpreted" by "would-be spoilers" opposed to the agreement.

"Also it could put some of these opposition groups, moderate opposition groups at risk," Toner said.

The spokesman did not elaborate, but reports suggest the agreement describes the location of US-backed rebel groups and could expose them to attack from rival factions.