"The battle to retake Tikrit will be difficult because of the preparations (IS) made," said Jawwad al-Etlebawi, spokesman for the Asaib Ahl al-Haq, a Shiite militia that is playing a major role in the operation alongside the regular army.
"They planted bombs on all the streets, buildings, bridges, everything. For this reason, our forces were stopped by these defensive preparations," Etlebawi told AFP.
"We need forces trained in urban warfare to break in," he said, adding that the jihadists are surrounded, "but any besieged person fights fiercely."
While Iraqi forces succeeded in surrounding the city and retaking towns across the Tigris river to the east, gaining ground inside it has proved much more difficult.
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Staff Lieutenant General Abdulwahab al-Saadi, the top army commander for Salaheddin, told AFP on Sunday that the forces attacking Tikrit needed air support from the US-led coalition.
Saadi said that he had asked the defence ministry to make the request to the coalition, but that no air support had been forthcoming.
IS spearheaded a sweeping offensive that overran large areas north and west of Baghdad last June.