Neither Russian nor Syrian planes bombed an aid convoy in Syria's Aleppo, Moscow said Tuesday, as outrage mounted over strikes which the Red Cross said killed around 20 people.

The United Nations suspended all humanitarian convoys in Syria on Tuesday following a deadly air strike on aid trucks, as fighting intensified after the regime declared an end to a week-long truce.

Air raids and shelling pounded key battlefronts in Syria on Tuesday, as outrage mounted over a strike on an aid convoy hours after Syria's military declared an end to a week-long truce.

The raid and renewed violence across the country dimmed hopes that the fraught ceasefire negotiated by Moscow and Washington could be revived.

U.S. Secretary of State John Kerry said he expected more aid deliveries to reach civilians in Syria on Monday but it remained to be seen if a ceasefire would hold.

The ceasefire "did pretty well last night, trucks are moving to maybe eight locations or more" to deliver aid, Kerry told reporters as he met Tunisia's foreign minister in New York.

Syria's President Bashar Assad vowed to retake all of Syria from "terrorists," hours before a truce brokered by Russia and the United States was due to take hold on Monday.

Iran on Sunday welcomed the proposed ceasefire in Syria brokered by Russia and the United States, but said a monitoring system was needed to stop it being exploited by "terrorists."

The United States and Russia, two former Cold War foes that have brokered a ceasefire deal for Syria, rely mostly on air raids in their separate military campaigns in the war-wracked country.

Here are key points about how these two powers are trying to fulfill their military objectives in Syria, where a bloody civil war has raged since 2011.

Syria's opposition was weighing whether to take part in a truce brokered by Russia and the United States due to start on Monday, after air strikes killed dozens in rebel-held areas.

The Syrian government has begun releasing 169 prisoners in exchange for the return of the bodies of five Russian soldiers killed when their helicopter was downed, a lawyer said Wednesday.

Hundreds of civilians on Wednesday began returning to the border town of Jarabulus in northern Syria, two weeks after pro-Ankara fighters recaptured it from Islamic State (IS) jihadists.