A new ceasefire in Syria brought a full day with no combat deaths in the war between President Bashar al-Assad and his opponents, a monitoring body said on Tuesday, and 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 U.N. 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 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.

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

Outside the scope of the truce, Turkey said on Tuesday that air strikes by a U.S.-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 Two.

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

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.

Dramatic Improvement

In Geneva, 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 U.N. convoys have yet to enter Syria, 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 U.N. was still waiting for Damascus to issue letters authorizing the deliveries. "We are eagerly hoping and expecting the government to issue them very soon."

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 neighborhood 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.

Two aid convoys, each of around 20 trucks, crossed into northern Syria from the Turkish border town of Cilvegozu, about 40 km (25 miles) west of Aleppo, a Reuters witness said, although with security a concern it was not clear how far into Syria they would go. A Turkish official said they carried mostly food and flour.

The Syrian government has said it would reject any aid deliveries to Aleppo not coordinated through itself and the U.N., particularly from Turkey.