Radiation from Japan’s quake-stricken Fukushima Daiichi nuclear plant has reached harmful levels, the government warned.

A third blast at the plant has damaged one of the reactors’ containment vessels for the first time and if it is breached, there are fears of more serious radioactive leaks.

Residents within 30km of the site have been warned to evacuate or stay indoors, as officials extended the danger zone, the BBC reported.

The crisis was sparked by a 9.0-magnitude quake and tsunami on Friday, which has officially killed 2400 people. However, estimates suggest at least 10,000 have died.

Thousands are still unaccounted for – including hundreds of tourists – while many remote towns and villages have not been reached.

More than 500,000 people have been made homeless.

The government has deployed 100,000 troops to lead the aid effort.

On Tuesday morning, reactor 2 became the third to explode in four days at the Fukushima Daiichi plant – 250km (155 miles) north-east of Tokyo.

A fire also briefly broke out at the plant’s reactor 4 on Tuesday and is believed to have led to radioactive leaks.

Reactor 4 had been shut down before the quake for maintenance but its spent nuclear fuel rods are still stored on the site.

In a televised address, Prime Minister Naoto Kan said: “There is still a very high risk of more radiation coming out.”

He added that the last remaining people within the existing 20km exclusion zone around the plant must leave.

Those living between 20km and 30km were also at risk and should not leave their homes.

“Now we are talking about levels that can impact human health,” said Chief Cabinet Secretary Yukio Edano.

He told residents: “Please do not go outside. Please stay indoors. Please close windows and make your homes airtight.

“Don’t turn on ventilators. Please hang your laundry indoors.”

A 30-km no-fly zone around the reactors has been imposed to prevent planes spreading the radiation further afield.

Radiation levels around Fukushima for one hour’s exposure rose to eight times the legal limit for exposure in one year, said the plant’s operator, the Tokyo Electric Power Co (Tepco).

The International Atomic Energy Agency said earlier that dose rates of up to 400 millisieverts per hour had been recorded at the power plant site, based on information received from the Japanese authorities.

Exposure to over 100 millisieverts a year is a level which can lead to cancer, according to the World Nuclear Association.

On Monday, a hydrogen blast at the Fukushima plant’s reactor 3 was felt 40km (25 miles) away. It followed a blast at reactor 1 on Saturday.

All explosions have followed cooling system breakdowns. Engineers are trying to prevent meltdowns by flooding the chambers of the nuclear reactors with seawater.

Japan’s nuclear safety agency said it suspects Tuesday’s blast may have damaged the vessel that holds reactor 2 – a more serious incident than the previous explosions, which were thought just to have damaged the buildings that housed the reactors.

The UK Foreign Office has updated its travel advice to warn against all non-essential travel to Tokyo and north-eastern Japan. British nationals and friends and relatives of those in Japan can contact the Foreign Office on +44(0) 20 7008 0000.