Russian war planes struck rebel-held areas north of Aleppo today as the army shelled the besieged old quarter in a major offensive, rebels and a monitoring group said.

Russia was reported yesterday to be sending more warplanes to Syria to ramp up its air campaign as the United States said it had not yet given up on finding a diplomatic resolution.

The latest strikes come 10 days into a Syrian government offensive to capture eastern Aleppo and crush the last urban stronghold of a revolt against Syrian President Bashar al-Assad that began in 2011.

Today's air strikes focused on major supply lines into rebel-held areas - the Castello Road and Malah district - while fighting raged in the Suleiman al Halabi neighbourhood, the front line to the north of Aleppo's Old City.

Rebels, led by the main Islamist rebel group Ahrar al-Sham, said today they had regained several areas in the Bustanal-Pasha district seized a day before, a strategic point northwest of the city.

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The army, aided by hundreds of Iranian-backed militias who have arrived in Aleppo, backed up the air campaign with a ground offensive on several frontlines to break rebel defences inside the city.

In a telephone conversation with US Secretary of State John Kerry yesterday, Russian Foreign Minister Sergei Lavrov said Moscow was ready to consider more ways to normalise the situation in Aleppo.

But Mr Lavrov criticised Washington's failure to separate moderate rebel groups from those the Russians call terrorists, which had allowed forces led by the group formerly known as the Nusra front to violate the US-Russian truce agreed on 9 September.

The United States made clear it would not, at least for now, carry through a threat made on Wednesday to halt the diplomacy if Russia did not take immediate steps to end the violence.

Moscow and Mr Assad spurned the ceasefire to launch the new offensive, potentially the biggest and most decisive battle of the civil war, which is now in its sixth year.

An army source quoted in state media said its forces had made advances, which was denied by rebels who said they had repelled a new assault.

A news commentary by the state-run Ikhbariyah said: "High level coordination from the air and ground by Syrian and Russian warplanes" had allowed the two allies to "successful hit locations where terrorist groups had dug in".

But rebels said Syrian troops backed by fresh reinforcements from Iranian-backed militias were struggling to make any gains in a ground offensive in a key frontline in the old city.