The United Nations Secretary-General has launched a scathing attack on the Syrian government and its allies, condemning the bombing of an aid convoy west of Aleppo.

Key points: Ban Ki-moon describes convoy bombers as cowards

Ban Ki-moon describes convoy bombers as cowards UN head blames Syria and also "powerful patrons who feed war machine"

UN head blames Syria and also "powerful patrons who feed war machine" UN now says convoy was destroyed in unspecified "attacks"

As outrage spanned the globe, aid agencies declared the attack a flagrant violation of the laws of war and suspended deliveries to assess its impact.

Meanwhile, the UN backtracked on describing the attack as air strikes, saying it did not have conclusive evidence about what happened.

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The incident, in which 18 trucks from a 31-vehicle convoy were destroyed on Monday evening, dealt another blow to the fragile week-old ceasefire.

In New York, UN Secretary-General, Ban Ki-moon, said it appeared the attack was deliberate.

"The United Nations has been forced to suspend aid convoys as a result of this outrage," Mr Ban said.

"The humanitarians delivering life saving aid were heroes. Those who bombed them were cowards."

Mr Ban blamed all the international players backing the warring sides, which would include Russia as well as the US, Turkey and Saudi Arabia.

"Powerful patrons that keep feeding the war machine also have blood on their hands," he said.

"Present in this hall today are representatives of governments that have ignored, facilitated, funded, participated in, or, even planned, and carried out atrocities inflicted by all sides of Syrian conflict against Syrian civilians."

But he singled out Syria for a stinging rebuke.

"Many groups have killed many innocents, but none more so than the government of Syria," Mr Ban said.

Destroyed aid trucks in Urm al-Kubra town, western Aleppo. ( Reuters: Ammar Abdullah )

Russia, Syria deny responsibility for attack

The UN, Red Cross and United States all described it as an air strike, implicitly pinning the blame on Russian or Syrian aircraft for breaking the ceasefire with a strike on a humanitarian target.

But Russia denied its aircraft or those of its Syrian Government allies were involved and said it believed the convoy was not struck from the air at all but caught fire because of some incident on the ground.

The local partner of the International Red Cross, the Syrian Arab Red Crescent (SARC) says its local director and around 20 civilians were killed.

Sorry, this video has expired Beverley O'Connor speaks to International Committee of the Red Cross's Ingy Sedky in Damascus

Ingy Sedky from the Red Cross in Damascus says staff have been hard hit by the killing.

"All our humanitarian aid in Syria is channelled through the SARC and we are on a daily basis in contact with that director and also the volunteers and its really heartbreaking and overwhelming to see some of the members dying while fulfilling their duties as humanitarians," she said.

The facts are still patchy but it appears the aid convoy left Aleppo in the morning, with the Syrian Government informed of the route and approving its passage.

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A rebel interviewed by Reuters says the convoy was escorted by rebel fighters who tried to shoot at aircraft circling overhead.

The convoy made it to the rebel-held town of Urem al Kubra and volunteers began unloading supplies for the 78,000 people living there before the attack began six or seven hours later.

"It was one of the regular convoys including wheat flour, education and nutritional supplies and also winter clothes and blankets and unfortunately most of the aid has been damaged which mean thousands of people in this area will be deprived," Ms Sedky said.

The Russian military says it had the convoy under surveillance, but broke that off after the convoy arrived.

It says neither it, nor the Syrian air force was responsible and suggested rebels or the White Helmets search and rescue group may be to blame.

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UN officials in Syria say they learned of the attack on the convoy while it was underway.

While they contacted all parties involved in the conflict, the strikes kept coming in, making it impossible to reach the dead and injured for hours.

After the Russian explanation, the UN put out a revised version of an earlier statement, removing wording on "air strikes" and replacing it with references to unspecified "attacks".

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UN humanitarian spokesman Jens Laerke said the references to air strikes in the original statement, attributed to the top UN humanitarian officials in the region and in Syria, were probably the result of a drafting error.

"We are not in a position to determine whether these were in fact air strikes. We are in a position to say that the convoy was attacked," he said.

UN raises possibility of war crime

Washington said it still believed the attacks were the result of an air strike, which could only have been carried out by Russia or the Syrian military.

Secretary of State John Kerry, who personally negotiated the ceasefire during months of diplomacy in the face of scepticism from other senior US officials, met 20 other foreign ministers including Russia's Sergei Lavrov.

"The ceasefire is not dead," Mr Kerry said.

"If this callous attack is found to be a deliberate targeting of humanitarians, it would amount to a war crime," UN aid chief Stephen O'Brien said in a statement.

ABC/Reuters

