FILE - In this Sunday, March. 3, 2013 file photo, Kurdish female members, foreground and background, of the Popular Protection Units stand guard at a check point near the northeastern city of Qamishli, Syria. With Sunni extremist fighters on the offensive against Kurdish areas in northern Syria using modern weapons recently captured from Iraqi forces, the Kurds' civil administration is putting forward plans to impose compulsory military service on residents in order for them to take part in protecting their land. (AP Photo/Manu Brabo, File)

WASHINGTON, Aug 8 (Reuters) - The Iraqi government provided a planeload of ammunition to Peshmerga fighters from Iraq's semiautonomous 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 acute 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 move that American officials hope will help the region's Peshmerga fighters 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. The hard-line fighters now appear set on trying to take the Kurdish capital.

The grave threat to Arbil, seat of the regional government and a hub for foreign firms in Iraq, appears to have at least temporarily eased a long-running feud between leaders of 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 sparred with Kurds 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 in their fight against militants.

The delivery of ammunition on Friday is sure to be welcome for Kurdish officials who for weeks have complained the Peshmerga, whose name means "those who face death," were overstretched and underequipped against the Islamist fighters, who have weapons seized from Iraqi army bases.

Both steps are significant in a country where in recent years Peshmerga and Iraqi forces under the command of Baghdad have been much closer to fighting each another than to cooperating.

'QUICKLY AS POSSIBLE'

The Obama administration is now working with the Iraqi government, the official said, to ensure additional requests from the Kurdistan Regional Government, for small arms and munitions including mortars and AK-47s, will be met soon.

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

While the shipment delivered on Friday came from existing Iraqi government stockpiles, the U.S. official said, it was not clear whether additional arms that Baghdad may provide to Kurdistan would come from Iraq's existing arsenal or would come from the United States via Baghdad.

The Obama administration has been reluctant to directly provide weapons to Iraqi Kurds because it would set a precedent for circumventing an allied government and would raise objections from Baghdad.

But the recent Islamic State advance has revealed worrying vulnerabilities of the Kurdish force. After many Iraqi soldiers abandoned their posts in the face of the initial Islamic State onslaught, the Peshmerga appeared much more battle-ready. 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 war that began in 2003, ensuring that hard-line militants cannot enter Arbil, the site of a U.S. consulate and a joint U.S.-Kurdish military operations center, is a priority.

U.S. officials, seeking to ensure the current campaign does not drag the United States into broader military action in Iraq, believe greater cooperation between Iraq's feuding ethnic and sectarian factions is critical to defeating the Islamic State. (Reporting by Missy Ryan; Editing by Mohammad Zargham and Eric Beech)