SBS World News Radio: US Secretary of State John Kerry has demanded Russia and the Syrian government immediately stop flights over Syrian battle zones in a "last chance" to salvage a collapsing ceasefire.
In an occasionally heated meeting at the United Nations Security Council, Mr Kerry suggested his Russian counterpart was living in a "parallel universe" when it came to addressing the conflict.
The rift between the United States and Russia over the war in Syria was on full display at UN headquarters in New York.
Two deadly attacks have effectively ended a ceasefire agreed between US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov.
The first saw 20 civilians killed when aid trucks from the Syrian Red Crescent were bombed.
The second has left up to five workers dead when a mobile medical unit was struck.
Responding to the widespread criticism of the first attack, which the US blames on Russia, Mr Lavrov cited various possible explanations.
"Many said it could have been a rocket or artillery shelling -- that's what the initial reports were. Then helicopters or warplanes were mentioned. So I think we need to refrain from emotional reactions and not make public comments. We must investigate and be very professional."
But an exasperated Mr Kerry decided an emotional approach was necessary.
"This attack has dealt a very heavy blow to our efforts to bring peace to Syria, and it raises a profound doubt about whether Russia and the Assad regime can or will live up to the obligations that they agreed to in Geneva."
The attacks prompted the UN to suspend aid convoys to priority areas but it has since said it will resume operations on a case-by-case basis.
Mr Kerry argues a ceasefire could still be salvaged and demanded planes be immediately grounded over areas where humanitarian aid would be delivered.
Setting aside his prepared remarks, and his usually reserved tone, Mr Kerry vented his frustration at leaders who say they want a ceasefire to work but haven't ensured Syrian President Bashar al-Assad complies with it.
"How can people go sit at a table with a regime that bombs hospitals and drops chlorine gas again, and again, and again, and again, and again, and again, and acts with impunity? Are we supposed to sit there and have happy talk in Geneva under those circumstances when you've signed up to a ceasefire and you don't adhere to it? What kind of credibility do you have with any of your people?"
Also in New York, Prime Minister Malcolm Turnbull has warned the situation in Syria is much bleaker than it was a year ago when he spoke with world leaders.
"I've spoken to many leaders in recent weeks about the Syrian situation and there is not a lot of optimism I regret to say. A political solution remains elusive. This conflict in Syria cannot continue to develop into, in effect, a proxy war."
But the war of words over the aid convoy attack continues.
Russia's defence ministry, having first suggested a fire on the ground had destroyed the convoy, now says a US combat drone had taken off from a Turkish airbase before appearing at the scene minutes before the incident.
Major-General Igor Konashenkov says the drone left the area after about 30 minutes but wouldn't draw any conclusions about its presence.
"Only the drone's owners know with what purposes this drone was in this area at the necessary moment or what tasks it was carrying out."
Meanwhile human rights groups say the Syrian town of Talbiseh has been hit by multiple airstrikes, killing two people.
Videos released by Syrian rebels appear to show parachute bombs descending.
A hospital worker is also seen holding two children as he screams for the outside world to help.
"Where is the international community? Where is the Islamic nation? Where are the Islamic scholars? Syria is being hit and Talbiseh is being demolished by bombs and missiles. Where is the world? Where is the Islamic nation? May God not forgive you."