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BAGHDAD, July 4 (Reuters) - U.S. forces backed by aircraft killed an estimated 25 gunmen during a clash north of the Iraqi capital Baghdad, the military said on Wednesday.

It said in a statement that the fighting took place during a three-day operation that ended on Monday near the town of Mukhisa in Diyala province, where 10,000 American and Iraqi troops are engaged in a major offensive against al Qaeda.

Gunmen attacked a U.S. patrol along the Diyala river, it said. Other guerrillas armed with rocket-propelled grenades joined the fight, prompting U.S. ground forces to call in air support, it added, without specifying the day of the clash.

It said an estimated 25 gunmen were killed. It was unclear if there were any American casualties.

Tens of thousands of U.S. and Iraqi soldiers are engaged in simultaneous offensives in and around Baghdad to deny al Qaeda territory in farmlands and towns from where they launch car bombs and other attacks at the capital and elsewhere.

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