About 140 people from the two countries joined the drill in Yokosuka, which the US Navy has chosen as the first city outside of the United States to host one of its nuclear-powered carriers in 2008.

"We have insisted that we would not be able to conduct a virtually useful drill without the participation of the central government and the US Navy," Yokosuka Mayor Ryoichi Kabaya told reporters after the exercise.

"Finally, it became a reality this year," he said.

Yokosuka has conducted nuclear disaster drills on its own, but the US Navy previously only would take part in the exercises as an observer, arguing such a disaster caused by its warships would be unrealistic.

The drill took place as the US Navy plans to replace the diesel USS Kitty Hawk in Yokosuka with the nuclear-powered USS George Washington next year, sparking angry protests from residents in the city of more than 420,000 people.

As the mock exercise got underway, the US Navy made an emergency call to the mayor warning of a possible leak of radioactive water from the US carrier.

The city's disaster management headquarters was alerted and experts headed out to take water and soil samples from the sea.

Opponents to the deployment of the new carrier said the mock drill should have involved a major radiation leakage and a more serious disaster.

But Rear Admiral James Kelly, commander of the US Naval Forces in Japan, disagreed.

"Great drill," Kelly told reporters. "Very important for us as members of this city."

"We have a very realistic scenario today, extremely realistic, based on 60 years of operating these kind of nuclear-powered warships and our naval reactors," he said.

The US Navy has said it must deploy the new carrier in Japan, its close ally in the region, because of the tense security situation in East Asia.

The United States, which has some 37,000 US troops in Japan, is shrinking and consolidating its military presence in the country, with plans to relocate some 8,000 Marines from Okinawa to Guam by 2014.

Japan's history as the only country to have been attacked with an atomic weapon -- it was bombed by the US in World War II at Hiroshima and Nagasaki -- has made the presence of nuclear warships controversial.

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