"No one was injured," the local Emergencies Ministry office said in a statement. "The radioactive levels at the plant and outside it are normal and absolutely harmless."

The Mayak plant, dubbed "Russia's ticking time-bomb" by environmentalists, suffered a series of accidents in 1949, 1957 and 1967 which were hushed up by Soviet governments.

Nuclear weapons and nuclear waste are reprocessed at the highly secretive plant, which is about 2,000 km (1,243 miles) east of Moscow. Foreigners are not usually allowed onto its territory because of its sensitive work with nuclear weapons.

Prosecutors said poorly implemented safety rules at the plant had allowed Thursday's leak to take place.

"After an investigation... it was found that the reason for the leak last Thursday of liquid radioactive waste at the Mayak plant was the result of severe violations of the safety rules," the local prosecutor-general's office said in a statement.

Emergency workers said they were notified at 1630 local time (1130 GMT) on Thursday and that they worked through the night to clear up the contaminated ground.

"The waste is stored far from populated places," the Emergencies Ministry said. "The situation is harmless for employees of the plant and residents of the nearest villages."

"We still have few details about this incident," said Vladimir Chuprov, head of Greenpeace's energy unit in Russia. "But we believe there are major systemic problems with Mayak."

Greenpeace says the plant is one of the most radioactive places on the planet and that local residents are still suffering from a 1957 accident at Mayak that exposed hundreds to radiation.

That accident was considered the worst nuclear disaster in the former Soviet Union until the 1986 Chernobyl disaster, the world's worst nuclear accident, which exposed the failings of Soviet nuclear management.

Nuclear officials say the Mayak plant has improved safety since the days of the Soviet Union.