Hailed by Japanese as anonymous heroes braving unknown dangers, about 300 engineers have been working around the clock to stabilize the six-reactor Fukushima complex since an earthquake and tsunami struck two weeks ago.

But they had to pull out of some parts of the complex, 240 km (150 miles) north of Tokyo, when three workers replacing a cable near reactor No. 3 were exposed to high contamination by standing in radioactive water on Thursday, officials said.

Two were taken to hospital with possible radiation burns after the water seeped over their boots.

"We should try to avoid delays as much as possible, but we also need to ensure that the people working there are safe," said Japanese nuclear agency official Hidehiko Nishiyama.

Safety fears at the plant and beyond - radiation particles have been found as far away as Iceland - are compounding Japan's worst crisis since World War Two.

As well as causing the most serious nuclear accident since Chernobyl in 1986, the March 11 quake and ensuing tsunami left about 27,400 people dead or missing across the northeast.

Despite increased radiation reports, fears of a catastrophic meltdown at the Fukushima plant are receding.

Two of the reactors are now regarded as safe in what is called a cold shutdown. Four remain volatile, emitting steam and smoke periodically, but work is advancing to restart water pumps needed to cool fuel rods inside those reactors.