IRBIL, Iraq — Iraqi forces resumed their offensive against the Islamic State in Mosul on Sunday after a weekslong pause, aiming to capture the militant-controlled western half of the city, where the tight, impassable streets and teeming neighborhoods will pose a formidable challenge to the advancing fighters. Prime Minister Haider al-Abadi announced the offensive in a televised address early Sunday, calling it “a dawn of victories.” As he spoke, units from Iraq’s army, federal police and state-sanctioned militias approached Mosul from the south and west.