The IAEA in Vienna said the three-man delegation will return to the agency's headquarters over the weekend and prepare a report on its findings.

The Japanese Government, which initially refused the IAEA's offer of help, is promising new laws to tighten nuclear site safety.

Current legislation does not fully cover the uranium-processing plant in Tokaimura, where the accident happened, because it is privately owned.

But a spokesman for the government says it wants to push ahead with new legislation to regulate nuclear facilities as soon as possible.

The BBC's Juliet Hindell: "It's unclear how close the team will be able to get"

The legislation will be based on the recommendations of Japan's Science and Technology Agency, which is conducting its own inquiry.

The IAEA team is not expected to visit the site until Friday. The plant is still too dangerous to enter, so it is unclear how close the team will get.

Three workers at the plant used steel buckets to pour 16kg of uranium into settling tanks, creating the conditions for a self-sustaining nuclear chain reaction.

It later emerged that the company had been using an illegal manual.

At least 49 people were exposed to dangerous levels of radiation in what was the biggest nuclear accident in the world since the Chernobyl disaster in 1986. Two workers remain in a critical condition.

IAEA spokesman David Kyd: "We have to respect the Japanese feelings"

The arrival of the IAEA team comes a day after the Japanese Government admitted a ventilator at the plant had been mistakenly left open for 12 days, allowing radioactive particles to leak into the atmosphere.

The ventilator was only turned off on Monday, three days after the radioactive substance iodine 131 was detected around the plant.