Retaking the city of Tikrit, where jihadists have rigged streets and buildings with explosives, will require "major sacrifices" on the part of Iraqi forces, a senior intelligence officer said Saturday.

Iraqi forces and allied paramilitaries have been fighting to retake the city since March 2, but halted ground operations for more than a week in what officials described as a bid to curb human and material losses before pushing forward again.

"The task of liberating Tikrit requires major sacrifices and street fighting, and our forces are ready for these sacrifices," the officer told AFP on condition of anonymity, indicating that the pause in operations only deferred the inevitable cost.

The Islamic State (IS) jihadist group has planted bombs in streets, rigged houses and other buildings with explosives, and built defensive works including berms and tunnels, also booby-trapped, the officer said.

IS spearheaded a sweeping offensive last June that overran much of Iraq's Sunni Arab heartland, and the operation to retake Tikrit is Baghdad's largest to date against the militants.

A US-led anti-IS coalition began carrying out air strikes in the Tikrit area on Wednesday, a move that increased available firepower in the air but has at least temporarily curbed it on the ground.

Key Iranian-backed militia forces that have done much of the heavy lifting in the drive to push IS back suspended offensive operations after the strikes began, commanders told AFP.

The Pentagon conditioned its intervention on an enhanced role for regular government forces and Friday hailed the withdrawal of "those Shiite militias who are linked to, infiltrated by, (or) otherwise under the influence of Iran".

Iran had been the most prominent foreign partner in the operation, but Baghdad eventually requested the US-led strikes after the drive stalled.

The battle has continued in the absence of the militia forces, with an army colonel saying there was heavy fighting on the southern outskirts of the city.

The advance was slow due to bombs planted by IS but security forces have gained some ground, the colonel said.