ERBIL, Iraq (Reuters) - Iraqi forces have advanced to 5 km (3 miles) from Mosul in an offensive against Islamic State's last major Iraq stronghold and there are signs of revolt against the group, the interior minister of the Kurdish regional government said on Saturday.
But he added that the battle is not expected to end soon.
Karim Sinjari, who is also acting defence minister in the area, told Reuters in an interview that Islamic State fighters - believed to number between 4,000 and 8,000 - will put up a fierce fight because of Mosul's symbolic value for the hardline Sunni jihadis.
"If they resist in the city, especially in old Mosul, it will be a big fight ... The roads are very thin, very narrow. You can’t have vehicles, you can’t have tanks. So it will be a fight, person by person," he said.
Islamic State leader Abu Bakr al-Baghdadi declared a caliphate and himself the leader of the world's Muslims at a Mosul mosque after seizing Iraq's second largest city in 2014.
"If Mosul is finished the caliphate they announced is finished. If they lose in Mosul, they will have no place, just Raqqa," Sinjari told Reuters in an interview.
The Syrian city of Raqqa is Islamic State's other major stronghold.
"They will have to go to Syria. They will be surrounded in one area."
The much-heralded battle to capture Mosul began last week and is expected to be the most important battle fought in Iraq since the U.S.-led invasion in 2003.
Iraq has come a long way since June 2014 when five Iraqi army divisions crumbled as Islamic State swept into Mosul. Islamic State has been dislodged from other major cities such as Falluja. That campaign lasted just over one month.
With air and ground support from the U.S.-led coalition, an Iraqi force of about 30,000, joined by U.S. special forces and under U.S., French and British air cover, is ready to push into Mosul after recapturing Falluja and Ramadi, west of Baghdad, and seizing the Sunni stronghold of Tikrit in central Iraq.
“I think it (the fight for Mosul) will be longer than Falluja and Tikrit, Mosul is a big city," Sinjari said.
So far, Kurdish forces have seized 20 villages and the Iraqi army have taken 10, he said.
It is not clear whether Baghdadi, an Iraqi who spent time in a U.S. military jail in Iraq, will risk death or capture and join his fighters in the battle for predominantly-Sunni Muslim Mosul, home to more than 1.5 million people.
"According to unconfirmed reports Abu Bakr was in Mosul three days ago. People saw him visiting fighters and encouraging them. We are not sure he was present, this is information," said Sinjari.
Iraqi forces would not be able to defeat Islamic State without help from the inside, such as informers or spies and cooperation from Sunni tribal groups, said Sinjari.
He said the jihadi group, which comprises former Sunni officers from Saddam Hussein's army, have built underground tunnels and dug a trench around Mosul which they filled with oil to set on fire when the offensive gets closer to the city.
"There are many reports that there are elements that have agreed to kill members of Daesh. Some members of Daesh were killed in the street," he said, using the Arabic acronym for Islamic State, which rose from the ashes of al-Qaeda in Iraq.
"They don’t want Daesh. Some were killed and some left. These are people who have weapons, who carry out attacks in specific areas at night and slip away."
Reuters could not independently verify these accounts.
"There are a lot of people who withdrew from the fight. They executed them," said Sinjari.
The plan, Sinjari said, is to surround Mosul from all sides.
The pressure may prompt Islamic State to become more ruthless, as previous military campaigns against the group suggest.
People who escaped from the jihadists in the town of Hawija paid a heavy price when they were caught on a road, said Sinjari.
"They killed 118 of them," he said. "We expect them to take (people for a) human shield, everything is expected from Daesh, he said.
Editing by Jeremy Gaunt