Iraqi forces are advancing into the centre of Ramadi, after launching a major assault to drive Islamic State militants from the city, officials say.

Security sources told the BBC that troops and allied tribesmen, backed by US-led air strikes, had already retaken two districts, and entered two others.

They are heading towards the main government complex, and have come up against snipers and suicide bombers.

Ramadi fell to IS in May in an embarrassing defeat for the Iraqi army.

Last month, government forces completed their encirclement of the predominantly Sunni Arab city, about 90km (55 miles) west of Baghdad, cutting off militants inside the centre from strongholds elsewhere in Anbar province and in neighbouring Syria.