British and Iraqi forces backed by helicopters killed 20 Shiite militiamen in the southern province of Maysan in coordinated raids on Monday that also left nearly 40 people wounded.
Troops called in air support after coming under heavy small-arms fire and rocket-propelled grenade attacks during the raids in the provincial capital Amara and Mujar al-Kabir, 30 kilometres (18 miles) south, officials said.
"Coalition forces killed at least 20 terrorists, wounded six suspected terrorists and detained one suspected terrorist during operations targeting secret cells operating in Amara and Mujar al-Kabir," a US military statement said.
The southern provinces of Iraq are under the control of British forces, which were involved in the raids, local security and health officials said.
"Iraqi special forces backed by multinational forces were involved in a operation in the Maysan province, but we are still awaiting details," British military spokesman Major David Gell said.
Amara health department director Jameel Mohammed confirmed receiving at least 16 bodies and another 37 wounded people.
The dead are believed to be Shiite militiamen loyal to radical cleric Moqtada al-Sadr who is a dominant force in Maysan province. His militants have often clashed with Iraqi and British troops in the region.
"Last night the British forces carried out a mission using helicopters and aircraft in Amara. They used bombs, missiles and small arms fire," said Sadr spokesman in Amara, Auda al-Baharani.
British forces pulled out of Amara in August 2006, handing over responsibility for the city's security to Iraqi security forces. But since then Amara has often seen clashes between militiamen and security forces.
"The individuals detained during the raid are believed to be members of the secret cell terrorist network known for facilitating transport of weapons and explosively formed penetrators from Iran to Iraq, as well as bringing militants from Iraq to Iran for training," it said.
The US military has regularly charged that these explosively formed pentrators are manufactured in Iran and smuggled to armed groups in Iraq to fight US-led troops.
"Coalition forces came under heavy small-arms fire and rocket-propelled grenade attacks in both Amara and Mujar al-Kabir," it said. "Using appropriate escalation of force measures, ground forces were forced to use close air support to suppress the enemy fire.
"Intelligence reports indicate that both Amara and Mujar al-Kabir are known safe havens and smuggling routes for secret cell terrorists who facilitate Iranian lethal aid," the military said.
"Reports further indicate that Iranian surrogates, or Iraqis that are liaisons for Iranian intelligence operatives into Iraq, use both Amara and Mujar al-Kabir as safe haven locations."
An Iraqi security official in Amara said the British troops threw leaflets from helicopters declaring: "The Iraqi government will not be soft on terrorism" and "Maysan will not be a safe area for the Iranian Qods Force and its agents who want to weaken the Iraqi government."
The leaflets also carried pictures of militants wanted by the security forces, he added.
In similar clashes in the southern city of Nasiriyah, four militants were killed by Iraqi security forces, a local police officer said.
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