BEIRUT, Sept 22 (Reuters) - Warplanes mounted the heaviest air strikes in months against rebel-held districts of the city of Aleppo overnight, as Russia and the Syrian government spurned a U.S. plea to halt flights, burying any hope for the revival of a doomed ceasefire.

Rebel officials and rescue workers said incendiary bombs were among the weapons that rained from the sky on the city. Hamza al-Khatib, the director of a hospital in the rebel-held east, told Reuters the death toll was 45.

Its as if the planes are trying to compensate for all the days they didnt drop bombs during the ceasefire, Ammar al-Selmo, the head of the civil defense rescue service in opposition-held eastern Aleppo told Reuters.

It was like there was coordination between the planes and the artillery shelling, because the shells were hitting the same locations that the planes hit, he said.

The assault, by aircraft from the Syrian government, its Russian allies or both, made clear that Moscow and Damascus had rejected a plea by U.S. Secretary of State John Kerry to halt flights so that aid could be delivered and a ceasefire salvaged.

In a tense televised exchange with Russian Foreign Minister Sergei Lavrov at the United Nations on Wednesday, Kerry said stopping the bombardment was the last chance to find a way out of the carnage.

President Bashar al-Assad meanwhile indicated he saw no quick end to the war, telling AP News it would drag on as long as it is part of a global conflict in which terrorists were backed by Saudi Arabia, Qatar, Turkey, and the United States.