The operator of the crippled Fukushima Daiichi nuclear power plant said on Thursday that radiation levels were stable after vapour was detected coming from one of the three reactors that went into meltdown after the triple disaster in Japan in March 2011.

Video images showed the vapour, which is thought to be steam, rising from the damaged building housing reactor No 3.

Tokyo Electric Power (Tepco), reeling from recent criticism of its handling of contaminated water at the plant, said the reactor's spent fuel pool was stable, adding that there had been no significant rises in radiation levels in the vicinity.

A worker monitoring live camera images noticed that what appeared to be steam was hovering just above the primary containment vessel at 8.20am on Thursday. The vapour was still visible two hours later, reports said.

Tepco said rainfall on Wednesday night could have been the cause.

"We think it's possible that rain made its way through the reactor building and having fallen on the primary containment vessel, which is hot, evaporated and created steam," said Mayumi Yoshida, a Tepco spokeswoman.

The reactor is one of three at Fukushima Daiichi that went into meltdown after a tsunami knocked out the plant's regular and emergency power supplies on the afternoon of 11 March 2011.

The building that houses reactor No 3 was also badly damaged by a hydrogen explosion in the aftermath of the tsunami.

Radiation levels near the top of the building are still too high for workers to enter. Instead, they are using remote-controlled machinery to remove rubble and tsunami debris.

Fuel inside the damaged reactors melted and is believed to be resting at the bottom of their primary containment vessels, where it is being cooled by huge quantities of water fed through a network of pipes and hoses.

Tepco has admitted it is struggling to create enough storage space at the site for contaminated water. In April, the firm requested permission to channel water containing comparatively low levels of radiation into the sea, but the move was opposed by local fishermen.

The vapour release comes soon after Japan's new nuclear regulation authority said it suspected that radioactive water used to cool the reactors was leaking into the Pacific ocean from underground.

Last week, workers discovered there had been a dramatic increase in levels of caesium and other radioactive substances in groundwater about 25 metres from the sea.