Iraqi forces on Friday battled jihadists making what looked increasingly like a last stand in Tikrit but the Islamic State group responded by vowing to expand its "caliphate".
Thousands of fighters surrounded a few hundred holdout IS militants, pounding their positions from the air but treading carefully to avoid the thousands of bombs littering the city centre.
Two days after units spearheading Baghdad's biggest anti-IS operation yet pushed deep into Tikrit, a police colonel claimed around 50 percent of the city was now back in government hands.
"We are surrounding the gunmen in the city centre. We're advancing slowly due to the great number of IEDs (improvised explosive devices)," he told AFP on condition of anonymity.
"We estimate there are 10,000 IEDs in the city," he said.
Massively outnumbered, the jihadists' defence consists of a network of booby traps, roadside bombs and snipers through which suicide attackers occasionally ram car bombs into enemy targets.
"Six soldiers were killed and 11 wounded in a suicide car bomb Friday morning in Al-Dyum neighbourhood" in western Tikrit, the colonel said.
An army major confirmed the death toll.
Tikrit was the hometown of dictator Saddam Hussein, remnants of whose Baath party collaborated with the jihadists when they took over almost a third of the country last June.
With crucial military backing from neighbouring Iran and a 60-nation US-led coalition, Baghdad has rolled back some of the losses.
It started with operations to secure the Shiite holy cities of Karbala and Najaf and bolster Baghdad's defences, then worked its way north, retaking Diyala province earlier this year.
Commanders see the recapture of overwhelmingly Sunni Arab Tikrit as a stepping stone for the reconquest of second city Mosul further north, which once had a population of 2 million.
IS has countered every military loss lately by ramping up its propaganda war with ever more shocking acts, such as getting a child to execute a prisoner on camera or destroying heritage sites.
On Thursday, the group released a recording presented as a speech by spokesman Abu Mohammed al-Adnani which marks the acceptance of a pledge of allegiance by Nigeria's jihadist group Boko Haram.
"We announce to you the good news of the expansion of the caliphate to West Africa," he said. Expansion is a pillar of IS doctrine and the group has recently declared new "provinces" in the Middle East and North Africa, albeit sometimes in places where it has a limited footprint.