Iraqi officials say troops are advancing into the city of Tikrit from several directions.
Islamic State fighters have reportedly fallen back into the city centre.
Tikrit is seen as a key launching pad for further operations to retake the parts of Iraq under I.S. control.
It's the biggest offensive launched against the Islamic State by Iraqi forces yet.
Wladimir Van Wilgenburg is with the Jamestown Foundation in Iraq and says there's cautious optimism.
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Wladimir Van Wilgenburg is with the Jamestown Foundation in Iraq and says after Tikrit, the next major offensive could be more complicated.
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