BAGHDAD (Reuters) - Iraq's military said it advanced through northern villages held by Islamic State on Wednesday, on its way to an airfield which could serve as the staging ground for a future offensive on Mosul, the biggest city held by the militants.

Army and counter-terrorism forces recaptured Telol al-Baj, about 260 km (160 miles) north of Baghdad on the main north-south road on Tuesday, a senior commander participating in the offensive told Reuters.

U.S.-led coalition air strikes have helped repel suicide car bomb attacks, the commander said. Both sides have suffered casualties, but most militants have fled into the desert, he added.

The troops are now around 45 km from the airbase at Qayara, less than two weeks after they set out from the refinery town of Baiji, military officials said. Retaking another refinery near Qayara with a production capacity of 16,000 barrels per day could also hit Islamic State's finances.

The military's recovery this month of Falluja, west of Baghdad, lent fresh momentum to the campaign to recapture Mosul, Iraq's largest northern city and the biggest anywhere within the jihadists' self-proclaimed caliphate spanning large stretches of territory seized in Iraq and Syria.

Two years since that land-grab, the tide has begun to turn against Islamic State as an array of forces lined up against it make inroads into their territory. Prime Minister Haider al-Abadi has pledged to retake Mosul this year.

Army troops advancing separately on the eastern side of the Tigris river from Qayara have made slower progress, taking a handful of villages since setting out from Makhmour more than three months ago.

Iraqi forces shelled Islamic State positions in Haj Ali on Wednesday in preparation for a ground advance that would allow them to reach the river bank, an army source said. They are expected to link up with the forces from the south near the air base, about 10 km west of the river.

Both offensives have so far displaced thousands of civilians from their homes, and the United Nations expects several thousand more to flee in coming days.

Upcoming military offensives in Iraq against Islamic State, including an assault on Mosul, could displace at least 2.3 million people, a U.N. official said last week.

(Reporting and writing by Stephen Kalin; Additional reporting by Isabel Coles)