Iraqi forces and militiamen are tightening their grip on Tikrit in an attempt to wrest the city from Islamic State militants.
Rebel gunmen and bombs have slowed progress as military and volunteer fighters push toward the center of the IS stronghold from two directions this week.
Commanders of the offensive say the operation, now in its second week, is advancing gradually and deliberately.
"We are engaging in a very delicate battle because ... we are facing booby-trapped terrain and sniper fire," a major general told the French news agency.
The leader of one of the largest Shi'ite militias, Hadi al-Ameri, said late Wednesday militants in Tikrit "have two choices, surrender or death."
Thousands of Iraqi troops teamed up with volunteer militias early last week to push the Islamic State front line back.
Map of Tikrit showing the direction of advance of Iraqi forces
About 200 kilometers from Baghdad, capturing Tikrit would give a geographic advantage to the government before an anticipated offensive against militants in Mosul in the coming months.
A U.S.-led coalition is conducting airstrikes against the extremist group that overran parts of Iraq and Syria in the past year, but the coalition has stayed out of the battle for Tikrit.
In a rare move since the air campaign against IS militants began in August, the U.S. military said the coalition carried out no airstrikes in Syria from Wednesday to Thursday. The partner countries focused on seven areas in Iraq, carrying out 13 strikes against IS targets with heavy emphasis on positions near Kirkuk and Fallujah.
Also on Thursday, Islamic State militants claimed a bomb attack on a police station in the Libyan capital Tripoli, lightly injuring an officer.