(Adds U.S. spokeswoman)

HILLA, Iraq, March 26 (Reuters) - Many people were killed or wounded by a U.S. air strike called to support Iraqi forces in the town of Hilla south of Baghdad on Wednesday, Iraqi security sources said.

U.S. forces confirmed the air strike and said they were not certain how many people had been killed but denied that there were large numbers of casualties.

One police source said at least 11 people were killed and 18 wounded in the strike, launched after Iraqi security forces called for support following street battles with Shi'ite militia members in the city's Thawra neighbourhood.

Another police source said 29 people were killed and 39 were wounded. He said six houses were destroyed in the strikes which lasted for an hour late on Wednesday evening.

Two other security sources said the combined total of dead and wounded was in the dozens, although they were unable to give precise casualty figures. All of the sources spoke under condition they not be named, as is usual practice in Iraq.

Major Allayne Conway, spokeswoman for U.S. forces south of Baghdad, said U.S. helicopters had responded to a call for help from SWAT special police units in Hilla.

"The Hilla SWAT guys were on the ground. They were engaged. Our attack helicopters were called in. They engaged," she said.

"We're still checking how many enemy personnel were killed. The initial report I had was four."

Iraqi security forces have battled Shi'ite militia in several southern cities and Shi'ite areas of Baghdad for the past two days. (Reporting by Sami al-Jumaily and Habib al-Zubaidy; writing by Peter Graff; editing by Sami Aboudi)

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