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Syrian President Bashar al-Assad on Thursday issued rebels in Aleppo a new threat that unless they agreed to a deal with the government, his forces would have "no option" but to expel them from the city.

The Syrian leader made the comments in an interview with Danish broadcaster TV2 aired two weeks after his forces announced an all-out offensive against the northern city.

UN special envoy for Syria Staffan de Mistura warned on Thursday that eastern parts of Aleppo could be "totally destroyed" before the year's end.

In his interview, Assad said the "best option" for Aleppo would be "reconciliations (like) in other areas," referring to towns and districts where opposition groups had agreed to local truces with the regime.

Otherwise, he said, he would "continue the fight with the rebels till they leave Aleppo.... There's no other option."

Syria's armed forces announced a large-scale assault on rebel territory in eastern parts of Aleppo on September 22.

Since then, government troops backed by Syrian and Russian warplanes have chipped away at opposition territory inside the city and on its outskirts.

The Syrian military said late Wednesday it would "reduce" airstrikes on rebel territory to allow civilians in the city to flee.

The aerial component of Assad's campaign had come under fierce international scrutiny in recent days, particularly after a series of airstrikes on hospitals in opposition-held quarters.

On Monday, the largest hospital in Aleppo's east was completely destroyed in bombardment.

But Assad denied his forces deliberately target medical infrastructure or restricted aid to civilians in the city.

"We never prevented any medical supply or food supply or any other thing from entering east Aleppo. There's no embargo, if that's what you mean," he said in the interview.

"As a government, we don't have a policy to destroy hospitals or schools or any such facility," he said, adding that such an attack would be "like shooting ourselves in the foot" because it would boost support for anti-regime groups.

But Assad also denied any non-jihadist rebel groups even existed in Syria.

"Do you know the unicorn, the animal that's like a horse, has a long horn? It's a myth. And the moderate opposition is a myth."

Agencies contributed to this report