A Syrian government offensive on rebel-held areas around Aleppo is threatening to derail UN efforts to persuade the warring parties to start peace talks.

Rebels said the new assault, backed by heavy Russian air strikes, was unprecedented and that the divided northern city might soon be encircled.

The UN special envoy declared on Monday that the talks in Geneva had begun.

But government and opposition officials contradicted Staffan de Mistura on Tuesday, as the fighting escalated.