Warplanes have launched fresh air strikes in Aleppo, hours after Syria's government announced an offensive to retake rebel-held areas of the city.

Syrian and Russian planes were pounding eastern Aleppo, according to activists. A rescue worker described what was happening as "annihilation".

The government has urged people to avoid positions held by rebels. Russia has not confirmed its involvement.

US-Russian talks to revive a collapsed truce have broken up without progress.

Russia supports the Syrian government, while the US backs the opposition. The two world powers accuse each other of failing to rein in their respective allies on the ground.