Officials say militants are now in control of some parts of Tikrit - Saddam Hussein's hometown which lies just 150km (95 miles) north of Baghdad.
Iraqi PM Nouri Maliki has vowed to fight back against the jihadists and punish those in the security forces who have deserted.
The insurgents who attacked Mosul are from the ISIS group.
It is not confirmed who is attacking Tikrit but one report said there was also fighting further south in Samarra.
ISIS - the Islamic State of Iraq and the Levant, which is also known as ISIL - is an offshoot of al-Qaeda.
It controls considerable territory in eastern Syria and western and central Iraq, in a campaign to set up a militant enclave straddling the border. (+ video) » | Wednesday, June 11, 2014
