Iraqi forces backed by Shiite militiamen and Sunni tribal fighters opened a large-scale operation Monday seeking to retake the Islamic State-held city of Tikrit, a critical stepping-stone for wider attempts to reclaim territory in northern Iraq.

The offensive — announced on Iraqi state television — marks the third attempt by government forces to regain control of the city from the Islamic State militants, who seized it in June.

Tikrit, about 110 miles northwest of Baghdad and the birthplace of Saddam Hussein, is a strategic foothold along a major highway linking the capital and Mosul, the largest city in northern Iraq. Mosul also is held by the Islamic State. […]