Members of the Kurdish peshmerga stand guard at a checkpoint at Tuz Khurmato village in Salahuddin Province June 26, 2014. REUTERS/Ahmed Jadallah WASHINGTON (Reuters) - The Iraqi government provided a planeload of ammunition to Peshmerga fighters from Iraq's semi autonomous Kurdish region on Friday, a U.S. official said, in an unprecedented act of military cooperation between Kurdish and Iraqi forces brought on by an urgent militant threat.

The official said Iraqi security forces flew a C-130 cargo plane loaded with mostly small-arms ammunition to Arbil, the capital of Iraqi Kurdistan, in a bid to strengthen the region's Peshmerga fighters as they struggle to keep militants from the Islamic State, an al Qaeda offshoot, at bay.

"This is unprecedented," the official said on condition of anonymity.

"Developments over the last few days have refocused the issue, and we've seen unprecedented cooperation between Baghdad and Arbil in terms of going after (the Islamic State), not only in terms of conversation but in terms of actual support."

In the first airstrikes in Iraq since U.S. forces withdrew in 2011, U.S. warplanes bombed Islamic State fighters several times on Friday, in an increasingly urgent attempt to halt the militants who have seized a wide swathe of territory since they swept into northern Iraq in June, and who now appear set on taking the Kurdish capital.

The acute threat to Arbil, seat of the regional government and a hub for foreign companies working in Iraq, appears to have at least temporarily eased a long-running feud between leaders in the Kurdistan region, who have long dreamed of an independent state, and the government of Iraqi Prime Minister Nuri al-Maliki, a Shi'ite Arab who has long feud with Kurdish leaders over land and oil.

As Islamic State fighters made another dramatic advance earlier this week, Maliki ordered his air force for the first time to back Kurdish forces to help them repel the militants.

The delivery of ammunition on Friday is sure to be a welcome development for Kurdish officials, who for weeks have complained that the regional fighters were overstretched and under equipped as they stared down the Islamist fighters, who have weapons they have seized from Iraqi army bases.

Both steps are significant in a country where in recent years the Peshmerga, whose name means "those who face death," and the Iraqi forces under the command of Baghdad have been much closer to fighting each other than to cooperating.

'QUICKLY AS POSSIBLE'

The official said the Obama administration was now working with the Iraqi government to ensure that additional requests from the Kurdistan Regional Government, for small arms and munitions including mortars and AK-47s, would be met in the near future.

"We're still coordinating with the government of Iraq to help fill the needs as quickly as possible," the official said.

The recent Islamic State advance has revealed the vulnerabilities of the Kurdish fighting force. After many Iraqi security forces abandoned their posts in the face of the initial Islamic State onslaught in June, the Peshmerga appeared to be much more battle-ready than their Iraqi counterparts. But the Peshmerga too have been routed in the past week.

While the Obama administration has been reluctant to return to military action in Iraq after the long, bloody U.S. war there, ensuring that hard-line militants cannot enter Arbil is a priority in part because a U.S. consulate and a joint U.S-Kurdish military operations center are there.

President Barack Obama told Americans in a late-night address on Thursday he was acting in part to "prevent a potential act of genocide."

U.S. planes also dropped relief supplies to religious minorities who have fled Islamic State militants in the mountains of northwest Iraq.

(Reporting by Missy Ryan; Editing by Mohammad Zargham and Eric Beech)