The Syrian army and its allies, backed by Russian air power, are keeping up a heavy bombardment of insurgent positions in and around Aleppo, rebels said on Monday, pressing to complete their recapture of the city's strategic southern gateway.

After reimposing a siege on rebel-held eastern Aleppo with an advance in the city's southwest on Sunday, the army and its allies now aim to block the insurgents from bringing in reinforcements, a Syrian military source said.

The city, Syria's largest before the war, has become the most important front for both Syrian President Bashar Assad and his allies and for the rebels seeking to bring him down.