The Islamic State group has suffered stinging defeats in the heart of its self-proclaimed "caliphate" recently, but its ultraviolent ideology has inspired attacks and recruits globally.
With IS brutality and population displacement reaching new highs, Washington sought increased powers from the US Congress to take on a group threatening to reshape the Middle East.
However, it was without direct support from the US-led coalition's air campaign that Iraqi government and allied forces punched into parts of Tikrit, marking a new phase in a 10-day drive to wrest the city back from IS.
A major general told AFP on condition of anonymity that government forces were battling "to cleanse the neighbourhood of Qadisiyah" in Tikrit.
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"But we are engaging in a very delicate battle because we are not facing fighters on the ground, we are facing booby-trapped terrain and sniper fire. Our movement is slow," he said.
An army colonel said forces coming from another direction had also retaken the main hospital on the city's southern edge.
The town of al-Alam, a flashpoint north of Tikrit along the Tigris river, was fully under the control of pro-government fighters and local anti-IS Sunni tribesmen Wednesday, an AFP reporter there said.
On the back foot in eastern and northern Iraq, IS tried to seize the initiative elsewhere, including with a spectacular coordinated attack in Ramadi in the west.
Twelve car bombs exploded almost simultaneously around the city after dawn, with at least seven suicide bombers targeting government security installations, police said.
Clashes ensued but IS failed to gain any ground in one of the biggest attacks against a rare pocket of government control in Anbar.