Iraqi forces made more progress Tuesday in their pushback against jihadists, but in Baghdad anger boiled over as hundreds stormed parliament over the fate of missing soldiers who surrendered in June.

After breaking a months-long jihadist siege by Islamic State (IS) fighters of the Shiite Turkmen town of Amerli, troops on Tuesday regained control of part of a key highway linking Baghdad to the north.

Two towns north of Amerli were also taken from the jihadists on Monday as Iraqi forces - backed by US air strikes - score their first major victories since the army's collapse across much of the north in June.

That collapse left some 1,700 soldiers in jihadist hands, with many believed to have been executed.

Demanding to know their fates, angry relatives stormed the parliament building in Baghdad, attacked MPs and began a sit-in in its main chamber, an official said.

Anti-riot police were trying to force out the hundreds of protesters, who were also calling for some officers to be held accountable, said the official, who was present at parliament.

The soldiers walked out of their base in Tikrit, north of the capital, believing a truce had been brokered. Instead, IS took them and later reported it had killed 1,700 soldiers, posting pictures of corpses online. There have been no independent reports on how many died.