US ambassador to Japan Caroline Kennedy toured the crippled Fukushima nuclear power plant for the first time Wednesday, pledging continued US help with the clean-up.

The 56-year-old envoy, who took up her post last November, was on a tour of Japan's northeast, which was devastated by the March 2011 earthquake and tsunami.

On Tuesday she met high school students and threw a ceremonial first pitch for a professional baseball match involving a local club in the city of Sendai.

Donning a white protective suit, helmet and mask, Kennedy saw the central control room for molten reactors at the plant, which has been releasing radiation since the disaster.

Kennedy, accompanied by her 21-year-old son John Schlossberg, was told how workers responded when the tsunami cut power supplies and halted cooling systems for the reactors.

"This is a very informative visit and I'm very grateful to all those people who are working here every day," Kennedy told reporters.

"The United States has always done all we can to support the people of Japan as they face this very, very tragic disaster," she said. "And we stand ready to help in any way we can in the future."

Kennedy, the lone surviving child of assassinated US president John F. Kennedy, was to spend one more day in the Fukushima region and inspect an offshore floating facility for wind power generation there.

The United States has been cooperating in decommissioning and cleaning up the power plant, some 220 kilometres (130 miles) northeast of Tokyo.

In a statement later, Kennedy said the US government would offer "our experience and capabilities, in particular, toward the near-term resolution of ongoing water contamination issues".

The plant's operator Tokyo Electric Power has long struggled to control waste water at the ravaged plant. The company poured thousands of tonnes of water onto reactors to keep them cool and continues to douse them.

"The United States looks forward to continuing a strong cooperative relationship with Japan in the energy security and clean energy arenas," the envoy said, "in addition to our ongoing assistance in the Fukushima region."