Baghdad, Iraq (CNN)Joint Iraqi forces have started what they believe will be the conclusive push to retake the Iraqi city of Tikrit from ISIS, a paramilitary force participating in the offensive said Tuesday. The forces have started "the decisive operation" to liberate Tikrit just over a week after the overall operation began, advancing toward the city from several directions, according to a statement from the predominantly Shiite paramilitary force Hashd Al-Shaab. ISIS wasn't making it easy, however. The Sunni extremist group blew up a key bridge near Tikrit, preventing the joint Iraqi forces from using it to cross the Tigris River to approach the city from the east. Iraqi Prime Minister Haider al-Abadi ordered Iraqi forces on March 1 to retake Tikrit and Salahuddin province. Tikrit, best known to Westerners as the birthplace of former Iraqi dictator Saddam Hussein, fell in June to ISIS, which has captured parts of Iraq and Syria for what it says is its Islamic caliphate. In the past few days, forces progressed roughly 50 miles (80 kilometers) down one road toward the city and, by Monday, were about a mile from its center. ISIS fighters have retreated toward the city center from frontline positions, Hashd Al-Shaab's media office said Tuesday. Al-Shaab's forces appear to be leading the fight for Tikrit, with support from the Iraqi army and Sunni tribesmen. Also assisting is Iran, which has provided advisers, weapons and ammunition to the Iraqi government. According to the Pentagon, there may be Iranians operating heavy artillery and rocket launchers as well. There have been several failed attempts to recapture Tikrit since the second half of 2014. If Iraq regains control of the city, it could mean that retaking Mosul -- a city 10 times bigger -- is possible. The Tikrit offensive involves around 30,000 fighters. CNN's Jason Hanna contributed to this report from Atlanta.
A major offensive for the city was launched just over a week ago . ISIS forces are retreating toward the center of Tikrit, says a Shiite force participating in the offensive .