A new ceasefire in Syria brought a full day with no deaths in the war between President Bashar al-Assad and his opponents, a monitoring body said, as efforts to deliver aid to besieged areas got cautiously underway.

Twenty-four hours after the truce took effect, the United Nations special envoy for Syria Staffan de Mistura declared the situation had improved dramatically, saying UN aid access should be possible soon, including to eastern Aleppo — the rebel-held half of the city which is under blockade.

The Syrian Observatory for Human Rights said that it had received not a single report of combatants or civilians killed by fighting in any areas covered by the truce.

The ceasefire, brokered by the United States and Russia, is supported by countries that back Mr Assad and his opponents, and marks the second attempt this year to halt a war that has frustrated all peace efforts since fighting began more than five years ago.

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It also marks the biggest bet yet by Washington that it can work with Moscow to end a war that Russian President Vladimir Putin transformed a year ago, when he sent warplanes to join the fight on Mr Assad's side.

Outside the scope of the truce, Turkey said on Tuesday that air strikes by a US-led coalition had killed three fighters from Islamic State.

Moscow and Washington have agreed to share targeting information for strikes against fighters from the jihadist group and the former Syrian branch of Al Qaeda, the first time the Cold War foes have fought together since World War II.

The agreement has been accepted by Mr Assad and, far more reluctantly, by most of the groups that oppose him.

The Observatory said the most intense fighting since the ceasefire began took place on Tuesday night in the village of Maan in Hama province.

Insurgents operating in the Hama area included jihadists and nationalist rebels fighting under the Free Syrian Army banner.

It was not immediately clear if the rebels involved in the fighting were parties to the ceasefire.

A Syrian military source said armed groups had broken the ceasefire at 6:00pm in Maan, attacking army positions with machine guns.

Observatory director Rami Abdulrahman said the fighting could be a serious threat to the ceasefire if it did not stop.

A senior US State Department official clarified that all groups except Nusra Front, which has renamed itself Jabhat Fateh al-Sham, and Islamic State had to abide by the agreement.

"Our current understanding is that the only groups that are eligible to be targeted during this period are IS and al-Nusra Front," the official told a conference call with reporters.

The international community's first goal is to deliver aid to civilians in Aleppo, Syria's biggest city before the war, which has been divided for years and where the opposition area is under siege.

'A significant drop in violence'

The ceasefire in Syria is said to be mostly holding. ( Reuters: Bassam Khabieh )

In Geneva, Mr de Mistura said there had been allegations of sporadic and geographically isolated incidents.

But he told reporters: "There is no doubt a significant drop in violence."

While stressing that only 24 hours of relative calm had yet passed, he said: "Sources on the ground, which do matter, including inside Aleppo city, said the situation has dramatically improved with no air strikes."

While UN convoys have yet to enter Syria, Mr de Mistura said that if the truce sticks, "[aid] access should be taking place very, very soon", and that the people of Syria could look forward to "no bombs and more trucks".

But he said the UN was still waiting for Damascus to issue letters authorising the deliveries.

"We are eagerly hoping and expecting the Government to issue them very soon," Mr de Mistura said.

Syrian state media said armed groups had violated the truce in a number of locations in Aleppo city and in the west Homs countryside on at least seven occasions on Tuesday.

The Observatory said pro-government forces had shelled near two villages in the south Aleppo countryside and a neighbourhood on the outskirts of Damascus.

But the reports of violence were far less intense than normal.

The Russian military, which sent reconnaissance equipment to detect and suppress attempts at violations, said the ceasefire had largely been observed in Aleppo.

Aid trucks cross into northern Syria from the Turkish border town of Cilvegozu. ( Reuters )

Two aid convoys, each of around 20 trucks, crossed into northern Syria from the Turkish border town of Cilvegozu, about 40 kilometres west of Aleppo, a Reuters witness said, although with security a concern it was not clear how far into Syria they would go.

Turkey had said it aimed, with the United Nations, to send trucks of food, children's clothes and toys to the northern Syrian city of Aleppo after the truce took effect late on Monday.

The United Nations said it did not own the aid trucks, and that it was still awaiting confirmation that the ceasefire was holding before sending in its own convoy.

Reuters