The Iraqi army has storrmed Qaraqosh, a Christian town under control of Islamic State since 2014, as part of operations to clear the entrances to Mosul, the militants' last major city stronghold in Iraq.
The advance took place as US Defence Secretary Ash Carter arrived on a visit to Baghdad to meet Prime Minister Haider al-Abadi and assess the campaign that started on Monday with air and ground support from the USled coalition.
A military statement said Iraqi army units entered the centre of Qaraqosh, about 20 kms (13 miles) southeast of Mosul, and were carrying out mop-up operations across the town which was emptied of its population in 2014, when Islamic State swept through the region.
Iraqi special units earlier this week captured Bartella, a Christian village north of Qaraqosh.
The offensive on Mosul is expected to become the biggest battle fought in Iraq since the invasion in 2003. Islamic State also controls parts of Syria.
The army is also trying to advance from the south and the east while Kurdish Peshmerga fighters are holding fronts in the east and north.
A Reuters photographer on the southern front saw plumes of smoke rising on Friday from a sulphur factory near that was under the control of Islamic State near the town of Qayyara, filling the air with toxic gasses.
It was not clear if the militants set it on fire to cover their retreat or if it was damaged during the fighting.
The army's media office said about 50 villages had been taken from the militants since Monday in operations to prepare the main thrust into the city of Mosul itself, where 5,000 to 6,000 are dug in, according to Iraqi military estimates.
"It's the beginning of the campaign. We do feel positively about how things have started off, particularly with the complicated nature of this operation," said a US official who briefed reporters ahead of Carter's trip to Baghdad.
Carter signalled during a visit to Ankara on Friday his support for a possible Turkish role in the campaign and said there was an agreement in principle between Baghdad and Ankara -- potentially ending a source of tension.
Officials said the details on any Turkish participation still needed to be worked out.
Roughly 5,000 US personnel are in Iraq. More than 100 of them are embedded with Iraqi and Kurdish Peshmerga forces involved with the Mosul offensive, advising commanders and helping ensure coalition air power hits the right targets.
US Navy Chief Petty Officer Jason Finan was killed on Thursday by a roadside bomb in northern Iraq as he was accompanying Iraqi forces, in the first US casualty of the Mosul campaign.
The militants retaliated to the advance of the Iraqi forces and the Kurdish fighters in Mosul by attacking Kirkuk yesterday, an oil city that lies east Hawija, a pocket they continue to control between Baghdad and Mosul.
Authorities in Kirkuk extended for a second day a curfew declared after the militants stormed police stations and other buildings in the city under control of Kurdish Peshmerga forces.
Iraqi Prime Minister Haider al-Abadi on Friday night ordered an army brigade to head to Kirkuk to assist the Peshmerga clear the remaining buildings still held by the militants.
Kurdish Peshmerga fighters took control of Kirkuk in 2014, after the Iraqi army withdrew from the region, fleeing an Islamic State advance through northern and western Iraq.
A total of 35 people have been killed since Friday in clashes of Kirkuk, including four Iranian technicians who were carrying maintenance work in a power station north of the city, according to a hospital source.
The toll does not include the jihadists who were killed or who blew themselves up during the fighting.
Kurdish leaders say they will never give up the ethnically mixed city, to which they, as well as Turkmen and Arabs, lay claim. Arabs complain that Kurds have since flooded to Kirkuk to tilt the demographic balance the other way.
Saddam Hussein ripped at the ethnic fabric of Kirkuk to ensure its dominance by Arabs, and not Kurds, Turkmen or Assyrian Christians who all see the city as part of their ancestral birthright.
Kurds say they are simply redressing historic wrongs perpetrated by Saddam. His policy of "Arabisation" in the north razed Kurdish villages and displaced hundreds of thousands.
