A permanent cut in the supply line would hamper IS's ability to move fighters and supplies between northern Iraq and Syria, two countries where the jihadists hold significant territory and have declared a "caliphate".

And retaking Sinjar -- where IS carried out a brutal campaign of killings, enslavement and rape against the Yazidi minority -- would also be an important symbolic victory.

Kurdish "peshmerga units successfully established blocking positions along Highway 47 and began clearing Sinjar," the US-led coalition against IS said in a statement, referring to the main route linking the jihadists' Iraqi hub of Mosul to Syria.

And the autonomous Kurdish region's security council (KRSC) also said the highway had been cut, and that multiple villages near Sinjar were retaken.

"The attack began at 7:00 am (0400 GMT), and the peshmerga forces advanced on several axes to liberate the centre of the Sinjar district," Major General Ezzeddine Saadun told AFP.

Huge columns of smoke rose over Sinjar as coalition strikes and Kurdish shelling targeted IS positions in the town.

Up to 7,500 Kurdish fighters are to take part in the operation, which aims to retake Sinjar "and establish a significant buffer zone to protect the (town) and its inhabitants from incoming artillery," the KRSC said.

"Coalition warplanes will provide close air support to peshmerga forces throughout the operation," it said.

The coalition carried out 24 strikes in the Sinjar area on Wednesday and eight more across the border in Syria's Al-Hol area.

Kurdish forces face an estimated 300 to 400 jihadists in the town, Captain Chance McCraw, a US military intelligence officer, told journalists in Baghdad.

But it is not just the jihadist fighters they will have to contend with: IS has had more than a year to build up networks of bombs, berms and other obstacles in Sinjar.

"This is part of the isolation of Mosul," the largest city in northern Iraq, Colonel Steve Warren, spokesman for the international operation against IS, said of the battle for Sinjar.

- 'Critical resupply route' -

"Sinjar sits astride Highway 47, which is a key and critical resupply route" for IS, Warren said in Baghdad.

"By seizing Sinjar, we'll be able to cut that line of communication, which we believe will constrict (IS's) ability to resupply themselves, and is a critical first step in the eventual liberation of Mosul."

The fact that the Sinjar operation comes at the same time as others against IS in Iraq and Syria also increases pressure on the group.

"It paralyses the enemy, right -- he's gotta make very tough decisions now on who does he reinforce," Warren said.

In conjunction with the Sinjar operation, fighters from the Syrian Democratic Forces group are battling IS across the border in the Al-Hol area.

And Syrian regime forces broke a year-long IS siege of a military air base in the country's north on Tuesday with backing from Russian air strikes.

After seizing Mosul and driving south toward Baghdad in a lightning offensive in June 2014, IS again turned its attention to northern Iraq, pushing Kurdish forces back toward their regional capital Arbil.

IS overran the Sinjar area in August 2014, attacking the Kurdish-speaking Yazidis in what the United Nations has described as a possible genocide.

Thousands of Yazidis fled to Mount Sinjar, which overlooks the town, and were trapped there by IS.

Aiding them was one of Washington's main justifications for starting its air campaign against IS last year.

International forces are also advising and training Iraqi forces, and American troops took part in a joint raid with the peshmerga last month in which a US soldier was killed.

With support from international strikes, Kurdish forces have regained significant ground from IS, and have been positioned on Mount Sinjar at the edge of town for months, with as little as 50 metres (yards) separating them from the militants.

But they had been concerned that retaking Sinjar would require a major deployment beyond it to protect the town from artillery fire.

"That's absolutely been addressed... There are enemy forces in towns south of Sinjar. We're gonna isolate those with fires," said Warren, referring to air strikes.