(Updates with US statement)

BAGHDAD, Aug 8 (Reuters) - U.S. forces said they killed 30 people in an air strike in Baghdad's crowded Shi'ite slum of Sadr City on Wednesday, describing those killed as militants linked to Iran.

Hospital officials put the death toll at at least 13. Hundreds of angry mourners later marched chanting through the streets of the slum after the raid on the eve of a major Shi'ite holy day.

The U.S. military said it, along with Iraqi military allies, had killed 30 "special groups cell terrorists" and captured 12 in a raid in the area. Helicopters opened fire after spotting militants in a vehicle trying to attack ground forces.

"Responding appropriately to the threat of the organised terrorist force, close air support was called and engaged the terrorist vehicle and organized terrorist force, killing an estimated 30 terrorists," the military said in a statement.

Reuters television pictures showed a U.S. attack helicopter circling above bullet-riddled cars and a house that showed damage from shrapnel and fire.

An official at Sadr City Hospital said it had received 10 bodies, while an official at Imam Ali Hospital, said it had taken in three bodies.

An official in the office of Shi'ite cleric Moqtada al-Sadr said U.S. aircraft had attacked Sadr City's Tareq neighbourhood before dawn.

(Additional reporting by Wisam Mohammed)

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