The remains of hundreds of mainly Shi'ite inmates killed by Islamic State militants when they overran a prison in northern Iraq more than two years ago have been unearthed by forces retaking the area from the group, a spokesman said.

Iraqi security forces on Thursday began the second phase of their offensive against Islamic State militants in Mosul, pushing into some eastern districts where the battle has been in deadlock for nearly a month.

Convoys of military vehicles loaded with Shi'ite paramilitary fighters hurtled across desert terrain in northwestern Iraq towards the Islamic State-held city of Tal Afar, chanting religious slogans and covering their faces to keep out the dust, Reuters reported.

Iraqi special forces said they pushed deeper into Mosul on Friday despite heavy resistance from Islamic State militants using civilians as cover, and were holding half a dozen city neighborhoods seized in the last 10 days.

Iraqi security forces advancing north along the Tigris river towards Mosul reached the last major town before the city on Friday, a police commander said.

Security forces battled for a second day Saturday with Islamic State group gunmen who infiltrated Kirkuk in a brazen raid that rattled Iraq as it ramped up an offensive to retake Mosul.