BAZWAYA: Iraq's special forces entered the outskirts of Mosul on Tuesday and were advancing toward its more urban center despite fierce resistance by Islamic State group fighters who hold the city, an Iraqi general said.

Troops have entered Gogjali, a neighbourhood inside Mosul's city limits, and were only 800 metres (yards) from the more central Karama district, according to Maj. Gen. Sami al-Aridi of the Iraqi special forces.

"The special forces have stormed in," he said. "Daesh is fighting back and have set up concrete blast walls to block off the Karama neighborhood and our troops' advance," he said, using the Arabic acronym for the Islamic State group.

Bombs have been laid along the road into the city, he added.

It was the first time Iraqi troops have set foot in Mosul in over two years, after they were driven out by a much smaller group of IS extremists in 2014.

Mosul is the final IS bastion in Iraq, the city from which it declared a "caliphate" stretching into Syria, and its loss would be a major defeat for the jihadis.

Yet entering Mosul could be the start of a grueling and slow operation for the troops, as they will be forced to engage in difficult, house-to-house fighting in urban areas.

The operation is expected to take weeks, if not months. The special forces troops remain some 5 miles (8 kilometres) from the center of the city, Iraq's second-largest.

The morning's action opened up with artillery, tank and machine gun fire on IS positions on the edge of the Gogjali neighbourhood, with the extremists responding with guided anti-tank missiles and small arms to block the advance.

Airstrikes by the U.S.-led coalition supporting the operation added to the fire hitting the district.

From the nearby village of Bazwaya, smoke could be seen rising from buildings on the city's edge, where shells and bombs were landing. The IS fighters quickly lit special fires to produce dark smoke in order to obscure the aerial view of the city.

Inside the village, white flags still hung from some buildings, put up a day earlier by residents eager to show they wouldn't resist Iraqi forces' advance. Some residents stood outside their homes, and children raised their hands with V-for-victory signs.

The families, estimated to number in the hundreds, will be evacuated from the village to a displaced persons camp, according to Brig. Gen. Haider Fadhil of the Iraqi special forces.

For over two weeks, Iraqi forces and their Kurdish allies, Sunni tribesmen and Shiite militias have been converging on Mosul from all directions to drive IS from the city.

Iraqi forces have made uneven progress in closing in on the city. Advances have been slower to the south, with government troops still 20 miles (35 kilometres) away.

To the north are Kurdish forces and Iraqi army units, and Shiite militias are sweeping toward the western approach in an attempt to cut off a final IS escape route.

The Shiite forces, Iran-backed troops known as the Popular Mobilisation Units, are not supposed to enter Mosul, given concerns that the battle for the Sunni-majority city could aggravate sectarian tensions.

Just behind the eastern front line, the army's ninth division has moved toward Mosul on the path cleared by the special forces, and was now approximately 5 kilometres (3 miles) from its eastern outskirts.

The U.S. military estimates IS has 3,000-5,000 fighters in Mosul and another 1,500-2,500 in its outer defensive belt. The total includes about 1,000 foreign fighters.

They stand against an anti-IS force that including army units, militarized police, special forces and Kurdish fighters totals over 40,000 men.

As the Mosul offensive has pressed on, bombings have continued in the capital, Baghdad, part of sustained IS efforts to destabilise the country. Dozens have been killed since the push on Mosul started in apparent retaliation attacks, mostly claimed by IS.

Also Tuesday, Kurdish authorities detained a Japanese freelance journalist covering the fighting. Japanese government spokesman Yoshihide Suga said in Tokyo that "we are aware that he is currently being detained" and that Japan is trying to determine why.

Japan's Kyodo News agency says that journalist Kosuke Tsuneoka was reporting on the battle to recapture the city of Mosul from the Islamic State group. Kyodo reported he is being held by the Kurdish militia known as the peshmerga.