Japan's use of oil for electricity generation has risen sharply since last year's devastating earthquake and tsunami forced its nuclear plants off-line, and could go higher still, the IEA said Wednesday.

The IEA said that in April, Japan needed 4.3 million barrels per day, up 6.1 percent compared with a year earlier after a gain of 13.1 percent in March as the country switched to fossil fuels to make up the nuclear power shortfall.

Earlier this month, Japanese Prime Minister Yoshihiko Noda called again for some nuclear plants to be restarted, pledging that safety would remain a priority after the quake devastated the Fukushima complex.

"Nuclear generation is an important power source (and) energy security is one of the country's most important issues," Noda said

Fukushima, the world's worst atomic disaster since Chernobyl, shook public confidence in the nuclear industry in Japan which had depended on it for some 30 percent of its energy needs.

The issue remains hugely sensitive -- Japan is the only country to have been attacked by nuclear weapons and the clean-up operation after Fukushima is expected to take decades.

The International Energy Agency, which advises developed countries on oil markets, said Japan used 500,000 bpd in April to produce electricity and this would likely increase to 600,000 or 700,000 bpd in coming months.

"Currently, we assume that two nuclear reactors located in the south ... will come back on-line in mid-July/early August but uncertainty remains" as Tokyo must clear the decision, the IEA said in its May monthly report.

The IEA said it assumes that nine nuclear units will be restarted by December, down from the 16 expected previously.

Its forecasts and Japan's ability to manage electricity supply during the hot summer months, when demand usually rises, will shortly be tested, the agency said.

"Oil demand could be revised higher if significant delays to nuclear restarts materialise," it added.