Al-Qosh (Iraq) (AFP) - Iraqi Kurdish fighters backed by US warplanes have retaken the country's largest dam from jihadists, Kurdish officials said, as the United States and Britain step up their military involvement.

The recapture of Mosul dam marks the biggest prize yet clawed back from Islamic State (IS) jihadists since they launched a major offensive in northern Iraq in early June, sweeping Iraqi security forces aside.

IS militants, who have declared a "caliphate" straddling vast areas of Iraq and Syria, also came under air attack in their Syrian stronghold of Raqa on Sunday, a monitoring group said.

US President Barack Obama told Congress that the "limited" airstrikes he has authorized on Iraq to retake its largest dam from militants protected US interests there.

The strikes have been conducted since Friday at the request of the Iraqi government, according to US National Security Council spokeswoman Caitlin Hayden.

Highlighting the stakes at hand, Obama said: "The failure of the Mosul dam could threaten the lives of large numbers of civilians, endanger US personnel and facilities, including the US Embassy in Baghdad, and prevent the Iraqi government from providing critical services to the Iraqi populace."

• 'Very extreme form of terrorism' -

British Prime Minister David Cameron described the Islamic State fighters sweeping across Syria and Iraq as a direct threat to Britain, and said all available tools most be used to halt their advance.

Cameron, writing in the Sunday Telegraph, said that while it would not be right to send an army into Iraq, some degree of military involvement was justified due to the threat that an expanding "terrorist state" would pose to Europe and its allies.

His Defence Minister Michael Fallon, in comments published Monday, said Britain's Iraq involvement now goes beyond a humanitarian mission and is set to last for months.

"We and other countries in Europe are determined to help the government of Iraq combat this new and very extreme form of terrorism," he was quoted as saying.

The Times reported that six British Tornado jets and a spy plane had begun flying beyond the Kurdish region to provide information on jihadists' movements that could be used in planning Iraqi military attacks in a "development that brings Britain closer to a direct combat role".

Two months of violence have brought Iraq to the brink of breakup, and world powers relieved by the exit of divisive premier Nuri al-Maliki are sending aid to the hundreds of thousands who have fled their homes as well as arms to the Kurdish peshmerga forces.

Buoyed by the US air strikes. Kurdish forces are fighting to win back ground they had lost since the start of August, when the jihadists went back on the offensive north, east and west of the city of Mosul, capturing the dam on August 7.

• 'Mosul dam is liberated completely' -

"Mosul Dam was liberated completely," Ali Awni, an official from Iraq's main Kurdish party, told AFP, a statement also confirmed by another party official and a Kurdish security forces officer.

The breakthrough came after US warplanes and drones pummelled the militants fighting against the Kurdish advance on Saturday and again on Sunday.

The US Central Command reported that the military had carried out 14 air strikes Sunday near the dam, which, located on the Tigris river, provides electricity and irrigation water for farming to much of the region.

CENTCOM said the strikes destroyed 10 IS armed vehicles, seven IS Humvees, two armoured personnel carriers and one IS checkpoint.

In Syria's Raqa province, the Syrian air force carried out 16 raids on the city of Raqa and several more on the town of Tabqa, killing at least 31 jihadists and eight civilians, said the Syrian Observatory for Human Rights.

"The regime wants to show the Americans that it is also capable of striking the IS," said the Britain-based group's director, Rami Abdel Rahman.

• Minorities targeted -

On another battlefront, security forces backed by Sunni Arab tribal militia made gains against the jihadists in Iraq's Anbar province, west of the provincial capital Ramadi, police said.

Fighting was also taking place near the strategic Euphrates Valley town of Haditha, located near another important dam, police Staff Major General Ahmed Sadag said.

In the north, members of minority groups including the Yazidis, Christians, Shabak and Turkmen, remain under threat of kidnapping or death at the hands of the jihadists.

The jihadist' storming of Sinjar on August 3 sent tens of thousands of civilians fleeing onto Mount Sinjar, prompting an international aid operation and helping to trigger the launch of US air strikes.

The Yazidis' non-Muslim faith is anathema to the Sunni extremists of IS.

Human rights groups and residents say IS fighters have been demanding that religious minorities in the Mosul region either convert or leave, unleashing violent reprisals on any who refuse.

Good for the Kurds, see what they can do with better weaponry and air support? Now, if they can lock down the Sinjar and the Zumara areas they will cut off western IS resupply routes from Syria to Mosul. Tal Afar and it's airbase isn't too far either. If the Kurds can take the airbase then they can resupply by air besides ground. They can move heavy equipment in too. Good to see more Sunni militias joining the fight in Anbar as well. Apparently in Syria there is word now too that the tribal leader of a major tribe has called on its 70,000 members to unite and fight IS after IS executed 700 members of the tribe. Syria is also now conducting air strikes in IS held territory as well.