Iraqi Kurdish forces say they now control much of the Mosul Dam in the north of the country, have wrested it away from Islamic jihadists.
The Peshmerga fighters were aided by US and Iraqi aircraft bombing militant targets.
The strategically important dam on the Tigris River supplies electricity and water for irrigation to much of Iraq.
Recapturing it would be a significant victory against the Islamic State group, but also remove the threat of the militants destroying the dam to create massive floods and destruction.
The Kurds say they met “fierce resistance” and there were increasing numbers of US air strikes through the weekend targeting jihadist vehicles.
The attacks by warplanes and drones have limited the mobility of the militants and helped the Peshmerga to advance.
The Kurds are also training volunteers from the Yazidi religious minority how to use assault rifles and rocket-propelled grenades to fight against Islamic State.
The training is said to be taking place at several camps in neighbouring Syria where thousands of Yazidi have fled to escape being massacred by the jihadists who consider them to be “devil worshippers”.