(Adds Iraqi officials confirming broad Baghdad clampdown)

By Dean Yates

BAGHDAD, June 18 (Reuters) - U.S.-led coalition forces killed at least 20 militants after coming under heavy attack from Shi'ite gunmen during raids in southern Iraq's Maysan province early on Monday, the U.S. military said.

An official in the office of anti-American Shi'ite cleric Moqtada al-Sadr in the Maysan capital Amara said 17 members of the firebrand's Mehdi Army had been killed and 45 wounded.

That would mark one of the deadliest incidents between U.S.-led forces and the Mehdi Army since a major security crackdown was launched in Iraq four months ago.

Iraqi authorities said major security operations were now under way targeting troubled belts around the capital. General David Petraeus, the top U.S. commander in Iraq, on Saturday said he had launched a fresh push against al-Qaeda militants.

Local officials said British and Iraqi forces were involved in the raids in Amara and the nearby town of Majjar al-Kabir, which the U.S. military said targeted weapon smuggling from neighbouring Iran into Iraq.

American forces also took part in the operation, said U.S. military spokesman Lieutenant-Colonel Christopher Garver, describing it as one of the biggest in months outside Baghdad against networks trying to smuggle in arms from Iran.

Maysan has a long border with Shi'ite Iran, which Washington accuses of arming, funding and training Shi'ite militias in Iraq. Tehran denies the charge.

During the operations, coalition forces came under heavy small-arms fire and rocket-propelled grenade attacks in both places, the U.S. military said in a statement.

"Using appropriate escalation of force measures, ground forces were forced to use close air support to suppress the enemy fire. During the close air support, at least 20 terrorists were killed," the statement said.

The official in Sadr's office in Amara said all the Mehdi Army members were killed in Majjar al-Kabir. The Mehdi Army militia is active across Iraq's south. It also has a stronghold in the Baghdad Shi'ite slum of Sadr City.

PERIODIC CLASHES

There have been periodic clashes between U.S. forces and the militia since a major security crackdown was launched in Baghdad and surrounding regions in February, but the militia for the most part has kept a low profile in the capital.

Latif al-Tamimi, head of the security committee on Maysan's provincial council, said 16 people including two women and a child had been killed and 45 wounded in the pre-dawn raids. He said 11 were killed in Majjar al-Kabir and five in Amara.

The U.S. military said that no women or children had been killed in the fighting.

Tamimi called the operation a "catastrophe", accusing troops of firing randomly. He said the Iraqi government should explain why the raids were carried out.

Ahmed Ali, a 32-year old engineer, said his cousin's home was attacked in Amara by British troops, who handed back security control of Maysan to Iraqi forces in April.

"They riddled the house with bullets and my cousin was killed. He is a traffic policeman and he has no relation with Mehdi Army," Ali said.

A British military spokesman said the raids were authorised by Iraqi Prime Minister Nuri al-Maliki and led by U.S. forces, with British troops playing a supporting role.

The U.S. statement said militants detained during the raids were believed to be members of a network known for transporting weapons and lethal roadside bombs known as explosively formed penetrators, or EFPs, from Iran into Iraq, as well as bringing militants from Iraq to Iran for training.

Overwhelmingly Shi'ite, Maysan, home to the Marsh Arabs and with large oil fields, has been spared much of the sectarian violence engulfing Baghdad and other areas that is pushing Iraq to the brink of all-out civil war.

Violence also flared in Baghdad after several days of relative quiet under a four-day curfew, with nine people killed and 25 wounded by two car bombs targeting motorists queuing for petrol in a southern district of the capital.

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