A senior military commander says Iraqi troops are facing stiff resistance from Islamic State militants as they push deeper into eastern Mosul.

Maj. Gen. Sami al-Aridi of the Iraqi special forces says his troops moved early Saturday morning into the Muharabeen and Ulama neighborhoods after fully liberating the adjacent Tahrir neighborhood on Friday.

Al-Aridi said IS militants are fighting back with snipers, rocket-propelled grenades and mortar rounds. Thick black columns of smoke were seen billowing from the two areas.

The offensive to retake IS-held Mosul, which was launched on Oct. 17, is the biggest military operation in Iraq since American troops left in 2011. If successful, the retaking of Mosul would be the strongest blow dealt to IS' self-styled caliphate stretching into Syria.