Obama made the offer in a telephone call with Japanese Prime Minister Naoto Kan, Edano told reporters.

"President Obama informed Prime Minister Kan that the United States was ready to provide further assistance including dispatching more nuclear experts and for mid and long term reconstruction efforts," Edano said.

US Ambassador to Japan John Roos said Wednesday that 34 nuclear experts had already arrived in the country carrying equipment to monitor radiation levels near the Fukushima No.1 plant, 250 kilometres (155 miles) northeast of Tokyo.

The experts are set to monitor the fluctuating levels from ground and air.

They join seven experts from the US Nuclear Regulatory Commission, two engineers and health hazards experts from the US Energy Department already in Japan in the wake of Friday's massive quake and tsunami.