The White House has said that President Barack Obama had informed Congress he authorised US air strikes in Iraq.
The move was to help retake control of the Mosul dam, which it said was consistent with his goal of protecting US citizens in the country.
Kurdish fighters pushed to retake Iraq's largest dam in an attempt to reverse gains by Islamic State (IS) insurgents who have overrun much of the country's north.
IS militants have seized several towns and oilfields as well as the Mosul dam in recent weeks, giving them the ability to flood cities or cut off water and electricity supplies.
Residents in the area were told to leave according to an engineer who works at the site, who also said militants told him they were planting roadside bombs along roads leading in and out of the facility.
US planes - deployed over Iraq for the first time since the US troop withdrawal in 2011 - has been striking targets near Mosul dam over the last 24 hours, peshmerga spokesman Halgurd Hikmat said.
US officials said last week the government was directly supplying weapons to Kurdish peshmerga fighters.
Witnesses said Kurdish forces have recaptured the mainly Christian towns of Batmaiya and Telasqaf, 30km from Mosul, the closest they have come to the city since IS insurgents drove government forces out in June.
The insurgents have also tightened their security checkpoints in Mosul, conducting more intensive inspections of vehicles and identification cards, witnesses said.
Earlier, British Prime Minister David Cameron said the advance of IS extremists posed a "clear danger" to the future safety of Britain's streets,
Body armour and counter-explosive equipment are among high-tech items being considered as part of a shipment of military hardware to be sent to Iraq to bolster anti-IS forces.
Mr Cameron said if IS succeeded in creating a wide-ranging caliphate encompassing several countries across the region "we would be facing a terrorist state on the shores of the Mediterranean and bordering a NATO member".
His comments came after the Church of England attacked the British government for having no "coherent or comprehensive approach" to combating the rise of Islamic extremism.
The continued threat from IS was underlined by reports of a fresh massacre of members of the Yazidi religious minority at the hands of jihadists despite continued US air strikes against militant positions.
Officials and eyewitnesses said 80 men were killed and their wives and children abducted.