A UN convoy delivering food relief in Syria has been destroyed in a deadly airstrike by Syrian or Russian planes.

The convoy of more than 30 truck was in the process of unpacking food and other care packages in the rural area of Urem al-Kubra near Aleppo when it was destroyed.

The UN said 18 trucks had been destroyed and 12 people killed in the airstrike.

The attack came just hours after the Syrian military declared an end to its observation of a week-old ceasefire agreed by the US and Russia.

The attack could only have been carried out by Bashar al-Assad's regime or his Russian allies, two high-ranking American officials told reporters.

"The Russians have the responsibility to refrain from taking such actions themselves but they also have the responsibility to keep the regime from doing it," one said.

"So either way, the burden is on the Russians to demonstrate quickly and in a significant way that they are committed to this process," the official added.

US Secretary of State John Kerry will attempt to speak to Russia's Foreign Minister Sergei Lavrov before Tuesday's meeting of the International Syria Support Group (ISSG), he said.

But if Moscow is unable to prove that it is serious about re-committing to what had been a seven-day lull in fighting, there may be no peace process to salvage, he warned.

"Our sense is that what happened today has dealt a serious blow to our efforts to bring peace to Syria," the other official said, speaking on condition of anonymity.

Russia and the United States agreed a deal during talks in Geneva earlier this month to pressure both Moscow's ally Assad and US-backed rebels to obey a ceasefire.

If the truce had lasted a week, the US and Russian militaries were to set up a joint targeting cell to hunt jihadist groups not party to the cessation of hostilities.

But Assad's forces continued to block shipments of aid to rebel-held areas, and on Monday declared an end to the truce, accusing the opposition of breaking it in turn.

Then the United Nations and monitors complained that a UN and Red Crescent aid convoy had been attacked, destroying at least 18 trucks and leaving 12 aid workers dead.

Kerry, Lavrov and ministers from the 23-nation ISSG are to meet Tuesday alongside the UN General Assembly in New York to decide what steps to take next for peace.

Kerry had earlier indicated that he wants to press on with the Russian-US process, but fellow US officials sounded more downbeat after the strike on the convoy.

"This was a difficult and trying day in Syria that I think raises very serious questions about whether the Russians can deliver their end of the arrangement," one said.