Syria's chief opposition coordinator Riad Hijab said on Tuesday that international efforts to achieve a ceasefire in Syria's civil war were doomed without any credible mechanism to designate blame or attribute consequences.

Hijab, once a prime minister under Syrian President Bashar al-Assad, said in an interview that the failings in the ceasefire deal brokered by the United States and Russia were flawed from the outset.

"We have no faith in the Russian side because their strategy is purely military," Hijab, who presides over the High Negotiations Committee of opponents of Assad, told Reuters on the sidelines of the annual United Nations General Assembly.

The ceasefire was shattered, and the United Nations suspended all aid shipments into Syria, after a deadly attack on a convoy carrying humanitarian supplies to a town near Aleppo on Monday.

"Without designating those responsible, we don't see any credibility to this cessation of hostilities," Hijab said. "We can see that with what happened to the attack on the convoy. There are no consequences. It is further evidence of the complete weakness of the international community."

Hijab said the opposition had information that proved Russia and Syria were behind the attack on the UN convoy. Both the Syrian and Russian governments have denied attacking the convoy and have blamed rebels.

"Nobody else has planes in that zone," Hijab said. He said a member of the High Negotiations Committee belonging to an armed opposition group had been assigned to protect the convoy and had witnessed Monday's attack.

The United States and Russia are on opposite sides of the 5-1/2-year-old war in which hundreds of thousands of people have been killed. Moscow backs Assad and Washington supports rebels seeking to topple him. Both countries share a commitment to defeat Islamic State militants who control parts of Syria and Iraq.

Hijab also took a swipe at the United States, saying he was disappointed in its approach. Echoing that view, senior opposition member Bassma Kodmani told Reuters that she hoped the United States would not sign up to a bad deal as part of rushed efforts by the Obama administration to push things forward.

"We’ve seen the Americans, they’re betting entirely on Russian good faith," she said.

A bad deal, she said, would be "no enforcement, no verification and agreeing to the terms of a political settlement which gives Assad any role. If the United States, this administration, takes that risk it will be committing a historic mistake."

Hijab said there would ultimately not be a military victory and that the opposition would continue political efforts to end the "suffering of the Syrian people." He acknowledged, however, that under current circumstances Assad's backer was using efforts to reach peace to get the upper hand on the ground.