Warplanes have 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.

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

US Secretary of State John Kerry demanded 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," Observatory director Rami Abdulrahman said.

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 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.

Sorry, this video has expired Activists report air raids in Aleppo using white phosphorous munitions

Iraqi army says it reached centre of key town near Mosul

Meanwhile in Iraq, government forces backed by air strikes from a US-led coalition seized the centre of Shirqat, a northern town seen as a stepping stone to recapturing Mosul from Islamic State.

The army, backed by local police and Sunni Muslim tribal fighters, were still clashing with the ultra hard-line jihadists after taking control of the mayor's office, the municipal building and the hospital, said a source from the Salahuddin Operations Command, which oversees military operations in the area.

Shirqat, on the Tigris river 100 kilometres south of Mosul, has been surrounded by Iraqi troops and Iranian-backed Shi'ite Muslim militias allied to the Government, but the militias so far have not participated in the operation.

Iraqi forces have advanced swiftly through the Shirqat area since Prime Minister Haider al-Abadi announced the operation on Tuesday morning.

The town's proximity to Iraqi supply lines reaching Qayyara airbase further north, which will be used as a logistics hub for the push on Mosul, lends it strategic importance.

A rocket attack on Tuesday that came within hundreds of meters of US forces at the base is being tested for chemical agents.

Remaining Islamic State fighters in Shirqat are resisting in groups of three and four from inside houses, according to the Iraqi operations command source, who said three army personnel had been killed in recent hours.

Tens of thousands of civilians were thought to be trapped in the area, which has been under Islamic State control since the group seized a third of Iraqi territory in 2014.

Iraqi authorities hope the course of the battle will allow most residents to shelter in place to avoid creating a humanitarian crisis as forces move towards Mosul, where more than a million people are still living.

US and Iraqi officials have said the push on Mosul could begin in October.

Reuters