US air strikes in support of Iraqi forces' efforts to retake the country's largest dam are aimed at protecting US interests there, President Obama says.
The failure of Mosul dam may put US staff and facilities, including the US embassy in Baghdad, at risk, Barack Obama warned Congress in a letter.
It comes after the US sent bombers for the first time to help Kurdish forces expel Islamic State (IS) militants.
Kurdish officials now say they have near complete control of the dam.
If the recapture is confirmed, it will be the biggest reverse for IS since they launched their offensive in Iraq in June.
The strategically important facility, seized by IS militants on August 7, supplies water and electricity to northern Iraq
President Obama notified Congress that the widened mission in Iraq would be limited in scope and duration, the White House said in a statement on Sunday.
In addition to citing concerns over US facilities in Iraq, he said the failure of the dam could "prevent the Iraqi government from providing critical services to the Iraqi populace".
It comes amid fears that IS, formerly known as Isis, could have used it to flood areas downstream.
The US military said it had conducted 14 strikes on Sunday, damaging 19 vehicles belonging to IS militants as well as a checkpoint near the dam.
Kurdish Peshmerga forces were still trying to clear mines and booby traps from the area round the dam on Sunday, a process which could take several hours, Kurdish officials said.
They said US special forces had been on the ground to help co-ordinate the air strikes.
IS militants in the Syrian province of Raqqa were also targeted in Syrian military air strikes on Sunday, the UK-based Syrian Observatory for Human Rights activist group said.
IS has seized a swathe of territory in recent months in Iraq and Syria, with thousands of Christians and Yazidis fleeing in the face of its advance.
Hoshyar Zebari, a Kurd who is Iraq's outgoing foreign minister, told the BBC that Peshmerga troops had encountered "fierce resistance" in the battle for the dam.
He said the next objective was to clear IS fighters from the Nineveh plain "to ensure the return of minorities".
Western states have continued to air lift humanitarian aid to refugees, many of whom have found shelter in the Kurdish region.
IS militants have been accused of massacring hundreds of people in areas under their control in northern Iraq and eastern Syria.
At least 80 men from the Yazidi religious minority are believed to have been killed, and women and children abducted, in a village in Iraq on Friday.
IS is also accused of killing 700 tribesmen opposing them in Syria's Deir Ezzor province, over a two-week period.
The violence has displaced an estimated 1.2 million people in Iraq alone.
Pursuing an extreme form of Sunni Islam, IS has persecuted non-Muslims such as Yazidis and Christians, as well as Shia Muslims, whom it regards as heretics.
Iraq's new Prime Minister, Haider al-Abadi, who is from the Shia majority, is grappling with the challenge of uniting the country against IS and winning back the trust of alienated Sunni Iraqis.