Radioactive contamination levels from Japan's crippled Fukushima nuclear plant have fallen sharply since the accident but will be "chronic and lasting" for many years, a French watchdog said Tuesday.

"The initial contamination linked to the accident has greatly declined," Didier Champion, crisis manager at the Institute for Radiological Protection and Nuclear Safety (IRSN), told reporters almost a year after the disaster.

"That doesn't mean that there won't be any more, far from it. Today, and for many years to come, we will have a situation of chronic and lasting contamination of the environment."

It was essential for Japan to maintain vigilant monitoring of fruit, milk, mushrooms, game and fish, Champion said.

"There are risks of chronic exposure at low dosage, and without care this can build up over time," he warned.

The March 11 catastrophe saw the plant swamped by a quake-generated tsunami that knocked out coolant pumps, triggered hydrogen explosions and caused three of its six reactors to suffer meltdowns of nuclear fuel.

Radioactive elements were spewed into the air by the blast and into the sea by cooling water that was pumped in in a desperate attempt to keep the overheated reactors under control.

The IRSN said the main radioactivity leaks occurred between March 12-25 in about 15 incidents, "of which the biggest probably took place before March 15".

It gave a provisional estimate that 408 peta-becquerels, or 408 million billion becquerels, of radioactive iodine had been emitted into the air.

This was 10 times lower than in the 1986 Chernobyl disaster in Ukraine, the world's worst nuclear accident.

The iodine releases posed a sharp but temporary hazard as the element quickly decays. A bigger problem, the IRSN said, was caesium-137, a long-lasting element which takes around 30 years to decay to half its level of radioactivity.

Caesium of all kinds released at Fukushima was estimated by the agency at 58 peta-becquerels, or three times less than Chernobyl. Caesium 137 accounted for 21 peta-becquerels.

Of around 24,000 square kilometers (9,200 square miles) of land contaminated by caesium 137, only 600 sq. kms (230 sq. miles) breached a safety threshold of 600,000 becquerels per square metre, the IRSN said.

This, again, was only a fraction of the territory contaminated by caesium after Chernobyl.

However, there remained "hot spots" of contamination, up to 250 kilometres (156 miles) away, where radioactive particles had been deposited by the weather.

So far, no death or cases of sickness have been directly linked to the disaster, IRSN said, stressing however that the impact on the civilian population over the long term, and on emergency workers and plant employees, remained unclear.