BEIRUT (Reuters) – Warplanes mounted the heaviest air strikes in months against rebel-held districts of the city of Aleppo overnight, rebel officials and the Syrian Observatory for Human Rights said on Thursday.

There was no immediate comment from the Syrian military on the reports, or mention of Aleppo air strikes on state media.

U.S. Secretary of State John Kerry demanded on Wednesday that Russia and the Syrian government immediately halt flights over Syrian battle zones, in what he called a last chance to salvage a collapsing ceasefire and find a way “out of the carnage”.

“It was the heaviest air strikes for months inside Aleppo city. It was very intense. In that area we didn’t see heavy fighting recently,” said Observatory Director Rami Abdulrahman.

Zakaria Malahifji, head of the Aleppo-based Fastaqim rebel faction’s political office, said it was the most intense bombardment since April. “There is no weapon they didn’t use,” he said, speaking to Reuters from Turkey.

A senior official in the Levant Front, another Aleppo-based rebel group, also said it was the most intense bombardment in many months. There were 15 raids alone targeting two areas where his group had a presence, he added.

The rebel officials said weapons used included incendiary bombs. “This is a type of pressure on the opposition. The Russians only want surrender. They have no other solution,” the Levant Front official said.

Much of the recent fighting in the Aleppo area has been in a district of military colleges and industrial sites on its southwestern outskirts, rather than inside the city itself.

(Reporting by Tom Perry and Angus McDowall; Writing by Tom Perry; Editing by Larry King and Pravin Char)