United Nations: A deadly air strike on a UN aid convoy by Syrian or Russian planes calls into question Moscow’s will to try to salvage a ceasefire, the United States said on Monday.

The attack could only have been carried out by Syrian President Bashar al-Assad’s regime or his Russian allies and Moscow must take responsibility either way, US officials said.

‘The United States is outraged by reports that a humanitarian aid convoy was bombed near Aleppo today,’ State Department spokesman John Kirby said.

Last week, the United States was forced to apologize after it weakened the ceasefire by bombing Syrian troops, but Washington said it had been accidental.

US officials said there could be no similar excuse from Russia for the targeting of non-combatant aid workers.

‘The destination of this convoy was known to the Syrian regime and the Russian Federation,’ Kirby said.

‘And yet these aid workers were killed in their attempt to provide relief to the Syrian people,’ he added.

‘Given the egregious violation of the cessation of hostilities we will reassess the future prospects for cooperation with Russia.’

Senior US officials went further, speaking on condition of anonymity.

‘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 try to speak to Russia’s Foreign Minister Sergei Lavrov before Tuesday’s meeting of the International Syria Support Group (ISSG).

But if Moscow is not serious about re-committing to what had been a seven-day lull in fighting, there may be no peace process to salvage, the senior official warned.

‘Our sense is that what happened today has dealt a serious blow to our efforts to bring peace to Syria,’ another US official said, also 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.