The gaping rift between the US and Russia over the war in Syria has been publicly exposed as the two countries accused the other of allowing a scrappy, days-old ceasefire to fall apart.

At an event that was supposed to enshrine the truce between various factions that was agreed on 9 September, US Secretary of State John Kerry and Russian foreign minister Sergey Lavrov, exchanged barbs and heated words at the UN Security Council on Wednesday morning. Mr Kerry suggested Mr Lavrov was living in a “parallel universe”.

The testy exchange, that took place as the world watched, came after two attacks on medical convoys. In one, on Monday, 20 civilians were killed when the Syrian Red Crescent convoy was struck. On Tuesday, an air strike in northern Syria killed five members of mobile medical emergency unit.

The attacks prompted a temporary suspension on UN overland aid convoys to priority areas in Syria, but the UN said on Wednesday it would be resuming operations on a case-by-case basis.

Sergey Lavrov leaves the chamber after listening to John Kerry’s comments (Reuters)

“Supposedly we all want the same goal. I’ve heard that again and again,” Mr Kerry told the council, according to the Associated Press. “But we are proving woefully inadequate in making that happen.”

He then outlined a series of US complaints Syrian President Bashar Assad’s government and its chief backer, Russia. He recited Moscow’s changing narrative over Monday’s attack on the an aid convoy that has included everything from claims of a justifiable counterterror strike to vehicles spontaneously combusting.

“This is not a joke,” said Mr Kerry, criticising those who engaged in word games to dodge responsibility over questions of “war and peace, life and death”.

Mr Kerry added: “To restore credibility, we must immediately ground all aircraft flying in those key areas in order to de-escalate the situation and give a chance for humanitarian assistance to flow unimpeded.”

The top American diplomat spoke just after Russian Foreign Minister Sergey Lavrov delivered his own set of barbs, underscoring the breakdown in trust in the 12 days since he and Mr Kerry clinched a cease-fire agreement and a potential US-Russian military partnership against Isis and Al-Qaeda.

Unlike Mr Kerry, who stressed the importance of Mr Assad’s government ending military operations against rebels and allowing in unfettered aid, Mr Lavrov said the US had the biggest responsibility.

“The key priority is to separate the opposition forces from the terrorists,” Mr Lavrov said.

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Responding to the wide criticism of the convoy attack, which American officials are blaming on Russia, Mr Lavrov cited various possible explanations.

Meanwhile, the Russian Defence Ministry yesterday said a US Predator drone was in the area when the aid convoy was hit and had appeared on the scene minutes before the incident.

Repeating denials of Russian involvement in the episode, Igor Konashenkov, a spokesman for the ministry, said Western allegations that Moscow was responsible were an attempt to distract attention from the US-led coalition's bombing of Syrian soldiers near Deir Azzor airport on Saturday.

In New York, Mr Kerry said at one point that listening to his Russian counterpart was like hearing about a “parallel universe”.

On Tuesday, the two diplomats had met with more than a dozen Arab and European foreign ministers, hoping to hold onto what might be salvageable from a week of relative calm in Syria. No one spoke of any progress, beyond a decision to hold follow-up discussions later this week.

“We are at a make or break moment,” said UN Secretary-General Ban Ki-moon said, as he opened the session.

His peace envoy to Syria, Staffan de Mistura, described his long-delayed plans for a multi-step Syrian peace process and transition that appeared, even by his own admission, untethered to reality on the ground. “I know, it looks like a dream,” Mr de Mistura said.

After the meeting, British Foreign Secretary Boris Johnson said Russia and Syria were the only two potential “culprits” and the UK had “doubts” that Syria would be able to carry out such a strike at night.

Mr Johnson told the BBC: “I think it's too early to say anything about criminality and that kind of thing, or indeed to make conclusive judgments about responsibility.