Media playback is unsupported on your device Media caption Watch: Residents are checked for radiation exposure in Koriyama

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

The warning comes after the plant was rocked by a third blast, which appears to have damaged one of the reactors' containment systems for the first time.

If it is breached, there are fears of more serious radioactive leaks.

Officials have extended the danger zone, warning residents within 30km (18 miles) to evacuate or stay indoors.

Analysis It appears that for the first time, the containment system around one of the Fukushima Daiichi reactors has been breached. Officials have referred to a possible crack in the suppression chamber of reactor 2 - a large doughnut-shaped structure below the reactor housing. That would allow steam, containing radioactive substances, to escape continuously. This is the most likely source of the high radioactivity readings seen near the site. Another possible source is the fire in reactor 4 building - believed to have started when a pool storing old fuel rods dried up. The readings at the site rose beyond safe limits - 400 millisieverts per hour (mSv/hr), when the average person's exposure is 3mSv in a year. A key question is whether this is just a transient spike, which might be expected if number 2 is the source, or whether the high levels are sustained. In the meantime, the key task for workers at the plant remains to get enough water into the reactors - and, now, into the spent fuel pools - with the poor resources at their disposal. Reactor breach worsens prospects

The government later said that radiation levels at the plant's main gate had fallen sharply.

The crisis has been prompted by last Friday's 9.0-magnitude quake and tsunami in north-eastern Japan.

On Tuesday morning, reactor 2 became the third to explode in four days at the Fukushima Daiichi plant.

A fire also briefly broke out at reactor 4, and is believed to have caused 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.

Chief Cabinet Secretary Yukio Edano said they were closely watching the remaining two reactors at the plant, 5 and 6, as they had begun overheating slightly.

He said cooling seawater was being pumped into reactors 1 and 3 - which were returning to normal - and into reactor 2, which remained unstable.

There was a hydrogen blast in reactor 3 on Monday, following another in reactor 1 on Saturday.

Radiation levels in the Japanese capital - 250km (155 miles) away - were reported to be higher than normal, but officials said there were no health dangers.

Flights cancelled

Tokyo residents have been stocking up on supplies, with some stores selling out of items such as food, water, face masks and candles.

Housewife Mariko Kawase, 34, told AFP news agency: "I am shopping now because we may not be able to go out due to the radiation."

Radiation levels in Chiba prefecture, next to Tokyo, were 10 times above normal levels, Kyodo News reports.

In other developments:

A 70-year-old woman has been rescued alive from rubble in the coastal town of Otsuchi, five days after the disaster

Airlines from Asia and Europe - including Germany's Lufthansa, Air China and Taiwan's EVA Airways - halted flights to Tokyo

The Nikkei share index tumbled again, ending 10.55% lower, as the central bank pumped almost $100bn (£62bn) more cash into the financial system, a day after its record $183bn intervention

Ninety-one countries have offered aid to Japan, ranging from blankets to search dogs and military transport aircraft

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

Homes sealed

He said that 140,000 people living within between 20km (12 mile) and 30km of the plant were at risk and should not leave their homes.

Media playback is unsupported on your device Media caption BBC Weather's wind-direction forecasts for Japan over coming days

Some 70,000 residents within 20km have already been evacuated, and the premier urged anyone left in that exclusion zone to leave.

"Now we are talking about levels that can impact human health," said the chief cabinet secretary.

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."

The government also announced a 30-km no-fly zone above the plant to prevent planes spreading the radiation further afield.

Rolling blackouts

After Tuesday's blast, radiation dosages of up to 400 millisieverts per hour were recorded at the site.

A single dose of 1,000 millisieverts causes temporary radiation sickness such as nausea and vomiting.

RADIATION AND CANCER Experts say even small radiation doses, as low as 100 millisieverts, can slightly raise cancer risk

Exposure to 1,000 millisieverts is estimated to increase risk of fatal cancer by about 5%

Leukaemia, a bone marrow cancer, is the most common radiation-induced cancer

Others include cancer of lung, skin, thyroid, breast and stomach; can take years to develop

Half of those exposed to between 4,000-5,000 millisiverts die in a month Q&A: Health effects of radiation

Rolling blackouts would affect five million households on Tuesday, said Tokyo Electric Power Co (Tepco), which runs the 40-year-old plant.

Japan's nuclear safety agency said earlier it suspected the latest blast may have damaged reactor 2's suppression chamber.

The BBC's Chris Hogg in Tokyo says that would make it a more serious incident than the previous explosions, which were thought just to have damaged the buildings housing the reactors.

The latest official death toll from the quake and tsunami stands at about 2,400 - but some estimates suggest at least 10,000 may have been killed.

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.

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.