By Dominic Evans
BAGHDAD (Reuters) - Iraq's armed forces, backed by Shi'ite militia, attacked Islamic State strongholds north of Baghdad on Monday as they launched an offensive to retake the city of Tikrit and the surrounding Sunni Muslim province of Salahuddin.
The offensive is the biggest military operation in the province since the Sunni Islamist radicals seized swaths of north Iraq last June and advanced toward the capital Baghdad.
Prime Minister Haider al-Abadi declared the start of the Salahuddin operations on Sunday during a visit to the government-held city of Samarra, where some of the thousands of troops and Shi'ite militia had gathered for the offensive.
The pace of their progress in Salahuddin will affect plans to recapture Mosul further north. A U.S. official said the assault on Mosul, the largest city under Islamic State control, could start as early as April but Iraqi officials have declined to confirm that timetable.
In Salahuddin, Islamic State fighters control several strongholds including Tikrit, hometown of executed former president Saddam Hussein and other Tigris river towns.
A source at the local military command said forces advanced north from Samarra toward the town of al-Dour, which officials describe as an Islamic State bastion, and Tikrit, which lies about 40 km (25 miles) north of Samarra.
Iraq's air force carried out strikes in support of the advancing ground forces who were being reinforced by troops and militia, known as Hashid Shaabi or Popular Mobilization units, from the neighboring eastern province of Diyala.
Artillery gunners in a military base just north of Tikrit also bombarded Islamic State positions in the city, another source said.
But the soldiers and Shi'ite militia fighters faced heavy opposition from Islamic State fighters who have had months to dig in, coming under attack as they advanced toward Samarra from their base to the east.
Sixteen of the advancing troops, five soldiers and 11 militia fighters were killed by gunfire and roadside bombs, army and medical sources said.
Also to the east of Samarra, a suicide bomber drove a vehicle packed with explosives into a convoy of Shi'ite militia fighters, killing four, police and hospital sources said. More than 30 militia fighters were wounded in clashes near al-Dour.
Declaring the start of operations on Sunday evening, Abadi gave Islamic State supporters what he said was one last chance to lay down their arms, or face "the punishment they deserve because they stood with terrorism".
But he also stressed that the army and militia must protect civilians and property in the battlefield. Shi'ite militia have been accused of mass executions and burning of homes in areas they have seized from Islamic State. Leaders of the paramilitary forces have denied the accusations.
Monday's offensive follows several failed attempts to drive the militants out of Tikrit since last June, when Islamic State declared a caliphate in the territories it controls in eastern Syria and northern and western Iraq.
In Iraq, months of U.S.-led air strikes, backed up by the Shi'ite militias, Kurdish peshmerga fighters and Iraqi soldiers have contained Islamic State and pushed them back from around Baghdad, the Kurdish north, and the eastern province of Diyala.
But they have held most of their strongholds in Salahuddin and taken new territory in the western province of Anbar.