French President Francois Hollande blamed the Syrian regime on Tuesday for the collapse of a U.S.-Russia-backed ceasefire and urged foreign backers of regime head Bashar al-Assad to help enforce peace or risk the country fragmenting.

“I say to the Syrian regime’s foreign backers that they must compel the regime to enforce peace otherwise they will bear the responsibility for the splitting up of the country and the chaos,” Hollande said in an address to the U.N. General Assembly in New York.

Russia and Iran are the main foreign backers of Assad’s regime. The French president also said the use of chemical weapons in Syria should not go unpunished.

Moreover, a deadly attack on a convoy carrying humanitarian supplies in the Aleppo area of Syria appeared to be hit by an air strike but it was not carried out by coalition jets.

“It was certainly not the coalition who struck from the air; it does look like an air strike,” U.S. Central Command spokesman Colonel John Thomas told reporters during a telephone briefing.

“The only other entities that fly in Syria are Russia and Syria,” he said.

The incident could deal a powerful blow to the ceasefire, the latest attempt to halt a war now in its sixth year.

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

The United Nations suspended all aid shipments into Syria on Tuesday after the attack. Thomas said the ceasefire in Syria was in jeopardy because of the increase in violence there.

“This is not the vision that I think was put in place more than seven days ago, to try to get humanitarian assistance flowing into the area and try to decrease the level of violence,” Thomas said.