The U.S. is expanding its air campaign in Iraq with attacks aimed at helping Iraqi forces fully regain control of the strategic Mosul dam.

The White House said President Barack Obama notified Congress on Sunday that the widened mission would be limited in duration and scope.

The administration's letter to Congress said "the mission is consistent with the president's directive that the U.S. military protect U.S. personnel and facilities in Iraq, since the failure of the Mosul Dam could threaten the lives of large numbers of civilians and threaten U.S. personnel and facilities — including the U.S. Embassy in Baghdad." It also noted that the failure of the dam could "prevent the Iraqi government from providing critical services" to the Iraqi people.

The letter said: "I am providing this report as part of my efforts to keep the Congress fully informed, consistent with the War Powers Resolution."

White House spokeswoman Caitlin Hayden said the operations are being undertaken in coordination with and at the request of the government of Iraq.

The latest round of U.S. airstrikes in Iraq against the Islamic State extremist group includes the first reported use of land-based bombers in the military campaign.

Kurdish security officials say Kurdish forces, aided by U.S. and Iraqi airstrikes, have now taken over parts of the dam. Islamic militants captured it less than two weeks ago, security officials said.

The U.S. began targeting fighters from the Islamic State with airstrikes Aug. 8, allowing Kurdish forces to fend off an advance on their regional capital of Irbil and to help tens of thousands of members of religious minorities escape the extremists' onslaught.

Recapturing the entire Mosul Dam and the territory surrounding its reservoir would be a significant victory against the Islamic State group, which has seized swaths of northern and western Iraq and northeastern Syria. The dam on the Tigris supplies electricity and water to a large part of the country.

A spokesman for the Kurdish forces, known as peshmerga, said the clashes were moving eastward.

"The west is in control of peshmerga. But there are some battles taking place in the (east) right now," said Halgurd Hekmat, peshmerga spokesman.

Mideast Iraq

A Kurdish peshmerga fighter patrols near the Mosul Dam at the town of Chamibarakat. (Khalid Mohammed/ Associated Press)

Another commander confirmed the information, saying that by Sunday evening, peshmerga forces had crossed the Tigris to the broad plains held by the Islamic State.

The U.S. military conducted 14 airstrikes Sunday, damaging or destroying 10 armed vehicles, seven Humvees, two armored personnel carriers and one checkpoint, according to a statement by the Central Command. On Saturday, it carried out nine airstrikes near the dam, destroying four armoured personnel carriers, seven armed vehicles, two Humvees and another armoured vehicle, the command said.

The peshmerga, the fighting force of the largely autonomous Kurdish region of northern Iraq, surrounded the Islamic State-held city of Tel Kayf after taking the nearby town of Tel Kasouf, said the commander, who spoke on condition of anonymity in line with military regulations.

Retreating ISIS fighters plant roadside bombs

The advance of Kurdish forces was hindered by roadside bombs and buildings rigged with explosives, planted by retreating Islamic State fighters, he said.

"They have reached inside the dam. There is no fighting, just the (roadside) bombs, and the abandoned buildings are all rigged with explosives," he said. "We will continue to advance and advance until we are given further instruction."

The commander said the evening advance occurred after the Iraqi government delivered 16 military Humvees, at least one with a mechanized bomb-disposal unit that was dismantling the roadside explosives.

Even as they advanced around the dam in northern Iraq, the commander said fighting forces were so poorly armed that he did not believe they could hold onto captured territory without a fast infusion of weapons — or continued U.S. airstrikes.

Mideast Iraq

Recapturing the dam would be a significant victory against the Islamic State group, which has seized vast swaths of northern and western Iraq and northeastern Syria, driving thousands of Yazidis, an ethnic minority group, from their homes. (Khalid Mohammed/Associated Press)

"We don't have the right weapons," he said.

Troubled relations between the Kurds and the central government in Baghdad have hindered the supply of arms to the force, leaving them overstretched and outgunned as the Islamic State group advanced.

Earlier this month, the militants swooped into Kurdish-held territory, seizing a border crossing and the Mosul dam. They also took control of villages around a northern mountain chain dominated by the Yazidis — adherents of an ancient faith seen as heretical by the militants.

The militants also took over villages near Irbil inhabited by Christians, and two Kurdish towns, Makhmour and Gweir.

Those seizures led to the flight of tens of thousands of Yazidis, Christians and Kurds into safer, Kurdish-held areas, and threatened to march onto Irbil itself.

Their advance was halted by U.S. airstrikes, the first American military involvement in Iraq since its troops pulled out in 2011. U.S. officials also said they would begin supplying weapons to Kurdish forces and said they had already been giving them some automatic rifles and ammunition.

U.S. delivers mounted machine guns 

But peshmerga forces say they haven't yet received any new supplies, and it's not clear how long the U.S. airstrikes will continue.

The senior commander said officers of his rank were told the U.S. had delivered mounted machine guns and rocket-propelled grenades to the Kurdish Defence Ministry, but they had not been distributed.

He said the weapons, while powerful, could not stop the advance of the Islamic State group, which has used heavily armoured U.S. Humvees taken from Iraqi troops who abandoned them in June when the militants first swept through the territory.

Echoing a complaint by other commanders, he said the peshmerga lacked the firepower to pierce Humvee armour, adding that they needed to be closer than 100 metres from the vehicle with an RPG in order to destroy it. He said the weapons mounted on Humvees typically allowed militants to stay back 400 metres.