Kurdish peshmerga fighters have fought their way to Iraq's Sinjar mountain and freed hundreds of people trapped there by Islamic State fighters, a Kurdish leader said on Thursday.

"The peshmerga have managed to reach the mountain. A vast area has been liberated," said Masrour Barzani, head of the Iraqi Kurdish region's national security council, adding that 100 Islamic State fighters had been killed.

"Now a corridor is open and hopefully the rest of the (Sinjar) region will be freed from Islamic State."

The assault, backed by US air strikes, ended the months-long ordeal of hundreds of people from Iraq's Yazidi religious minority, who had been besieged on the mountain since Islamic State stormed Sinjar and other Kurdish-controlled parts of northern Iraq in August.

"All those Yazidis that were trapped on the mountain are now free," Barzani said.

The peshmerga had not yet begun to evacuate them, he added.

Kurdish peshmerga soldiers began their offensive on Wednesday to break the jihadists' siege of the mountain and the town of Sinjar.

The peshmerga advanced from Zumar, east of Sinjar, capturing back 700 sq km over two days. US fighter planes carried out 45 strikes in support of Kurdish fighters on Wednesday, in addition to two strikes near Sinjar.

The impact of the air strikes was evident on Thursday. In one village called Little Koban, the bodies of five militants lay in a wadi.

The peshmerga said the Islamic State fighters had been trying to take cover from the air strikes.

"It's the best feeling to kill the enemy," said a peshmerga who took a photo of himself with a corpse in the background on his cell phone. "Look at his beard, the son of a bitch."

The words "Property of Islamic State" had been sprayed on houses in a nearby village.

The Kurds have yet to take back the actual town of Sinjar, but the freeing of the Yazidis from the mountain is a victory for the Kurds after Islamic State's routing of peshmerga fighters this summer.

The August spectacle of Islamic State fighters racing towards Arbil and the pleas of Yazidis trapped on Sinjar mountain, with thousands of others captured or killed, galvanised US President Barack Obama to military action.

Since then, Kurdish peshmerga forces have regained most of the ground they lost to Islamic State in northern Iraq, but Sinjar's awkward geography, out on a limb to the west, has made it difficult to penetrate. - Reuters