GENEVA/BEIRUT - The United Nations suspended all aid shipments into Syria on Tuesday after a deadly attack on a convoy carrying humanitarian supplies, as a week-old US-Russian sponsored ceasefire collapsed in renewed violence.

Washington said it was “outraged” by the apparent air strike that hit a 31-truck aid convoy late on Monday.

Russia, which is allied to the government of President Bashar al-Assad, denied that either its air force or that of the Syrian armed forces were responsible. The Syrian army also denied blame.

Moscow said only insurgents knew the full whereabouts of the convoy, but this contradicted the United Nations, which said all parties had been notified and the trucks were clearly marked.

The Syrian Red Crescent said the head of one of its local offices and “around 20 civilians” were killed.

The strike on the aid convoy appeared to deliver a death blow to the ceasefire, the latest failed attempt to halt a war, now in its sixth year, which has killed hundreds of thousands of people and created the world’s worst refugee crisis.

Syria’s army declared the ceasefire over on Monday, hours before the strike. While the United States initially said it was still hopeful of extending the truce, US officials acknowledged in the wake of the attack that there might no longer be any agreement left to salvage.

That would most likely wreck the last hope of any breakthrough on Syria before the administration of President Barack Obama leaves office in January, meaning his successor will inherit a war that has split the Middle East on sectarian lines and drawn in global and regional powers.

The ceasefire was meant to halt all fighting and allow aid to reach besieged areas, at a time when pro-government forces, with Russian and Iranian military support, are in their strongest positions for years and civilians in many rebel-held are completely cut off from food and medical supplies. “As an immediate security measure, other convoy movements in Syria have been suspended for the time being pending further assessment of the security sitaution,” Jens Laercke, a UN humanitarian aid spokesman, told a briefing.

The attack on the convoy of the Syrian Arab Red Crescent destroyed 18 of 31 trucks.

“If this callous attack is found to be a deliberate targetting of humanitarians, it would amount to a war crime,” UN aid chief Stephen O’Brien said in a statement. Peter Maurer, president of the International Committee of the Red Cross (ICRC) called it a “flagrant violation of international humanitarian law”.

While there were contradictory reports of the full death toll, one of those killed was the head of the Syrian Red Crescent for the area, Omar Barakat. The team on the ground was “in shock,” said ICRC’s Middle East chief, Robert Mardini.

The United Nations had only just received permission from the Syrian government to deliver aid to all besieged areas in the country, and had notified Washington and Moscow of the convoy’s route.

A local resident told Reuters by telephone that the trucks were hit by about five missile strikes while parked in a centre belonging to the Syrian Red Crescent in Urem al-Kubra, a town near Aleppo.

At the time of the strike, US and Russian officials had been meeting behind closed doors in Geneva to discuss extending the ceasefire, although Syria’s army had already declared the truce over and announced it would resume fighting.

Washington was “outraged”, said US State Department spokesman John Kirby. “The destination of this convoy was known to the Syrian regime and the Russian federation and yet these aid workers were killed in their attempt to provide relief to the Syrian people,” he said in a statement.

Kirby said Washington would raise the issue with Moscow and reassess the future of its cooperation with Russia.

Kremlin spokesman Dmitry Peskov said it was unlikely the ceasefire could be salvaged, and blamed Washington for failing to fulfil its obligation to separate militant fighters from insurgent groups covered by the truce.

A Russian Defence Ministry spokesman, Igor Konashenkov, said the Russian and Syrian air forces were not involved, and appeared to blame the insurgents: “All information on the whereabouts of the convoy was available only to the militants controlling these areas.”

Syria’s state news agency SANA quoted a source in the army as saying: “There is no truth to reports carried by some media outlets that the Syrian Arab Army targeted a humanitarian aid convoy in the Aleppo countryside.”

After five years of fighting that made a mockery of all peacekeeping efforts, the ceasefire deal was a gamble on unprecedented cooperation between the United States and Russia. Trust between the two Cold War-era foes is at its lowest point for decades.

The deal called for Washington and Moscow, which support opposite sides in the war between Assad’s government and insurgents but are both fighting against Islamic State militants, to eventually share targetting information, the first time they would have fought openly together since World War Two.

It was negotiated by Secretary of State John Kerry during months of intensive diplomacy, despite scepticism from other senior officials in the Obama administration.

Following the attack, a senior Obama administration official said of the ceasefire: “We don’t know if it can be salvaged.”

“At this point the Russians have to demonstrate very quickly their seriousness of purpose because otherwise there will be nothing to extend and nothing to salvage,” the official, who spoke to reporters on condition of anonymity, added.

Kerry, in New York for a UN summit, said the future of the agreement would depend on Moscow.

Foreign ministers from 20 nations emerged with differing views after a meeting to explore how to revive the ceasefire shattered by a strike on a humanitarian aid convoy on Monday, with one minister asking if matters were already hopeless.

The United Nations suspended all aid shipments into Syria after Monday’s deadly attack on a convoy carrying humanitarian supplies to a town near Aleppo, as a week-old US-Russian sponsored ceasefire collapsed in renewed violence.

“The ceasefire is not dead,” US Secretary of State John Kerry told reporters as he emerged from the gathering of the International Syria Support Group together with Russian Foreign Minister Sergei Lavrov.

The United States and Russia are on opposite sides of the 5-1/2-year-old civil war, with Moscow backing Syrian President Bashar al-Assad and Washington supporting rebels seeking to topple him. Both countries share a commitment to defeat Islamic State militants.

The Syrian Red Crescent said the head of one of its local offices and “around 20 civilians” were killed in Monday’s strike, which a war monitoring group blamed on Russian or Syrian aircraft.

Russia, which is allied to Assad’s government, denied that either its air force or that of the Syrian armed forces was responsible. The Syrian army also denied blame.

German Foreign Minister Frank-Walter Steinmeier struck a decidedly pessimistic note about the chances of halting violence in the Syrian civil war, now in its sixth year, as he arrived for the meeting on the sidelines of the UN General Assembly.

“We will have to reflect if there are ways back to negotiations on a truce, or if this has already become hopeless,” Steinmeier told reporters before the meeting in a New York luxury hotel.

Speaking afterwards, French Foreign Minister Jean-Marc Ayrault also voiced doubts.

“It was a dramatic meeting. Is there still a chance this ceasefire will be effective? I can’t answer that question,” Ayrault told reporters. “Without (a) ceasefire it will be (a) spiral of war, but we have to be honest, the US-Russian negotiation has reached its limits.”