BAGHDAD—Iraqi security forces were locked in a standoff outside the city of Tikrit on Sunday morning, local security officials said, as the military's most muscular effort to beat back a three-week-old Sunni insurgency appeared to stall.

The military offensive, which began Saturday afternoon after weeks of preparation, was hampered by extensive land-mine formations laid by insurgents on the main road between Samarra, the provisional headquarters of Iraqi forces north of Baghdad, and Tikrit, according to security forces at...