Hong King - The Japanese stock market sank again Thursday morning and the yen hit a record high against the U.S. dollar after a U.S. nuclear official warned that the situation at a damaged reactor was more serious than Tokyo has acknowledged.

The benchmark Nikkei 225 index dropped 2.5 percent within an hour of the open, wiping out much of a rebound staged during the previous day and returning toward the lows plumbed during a massive, panicky sell-off on Tuesday.

The broader Topix index sagged 3.2 percent.

The latest declines came as the barrage of ominous news about the reactors at Fukushima Daiichi Nuclear Power Station continued.

Late on Wednesday in Congressional testimony, Gregory Jaczko, the chairman of the U.S. Nuclear Regulatory Commission, gave the Obama's administration first assessment of the condition of the plant, apparently mixing information it has received from Japan with data it has collected independently.

Mr. Jaczko asserted that there was now little or no water in the pool storing spent nuclear fuel at the No. 4 reactor of the Fukushima Daiichi complex, leaving fuel rods stored there exposed and bleeding radiation into the atmosphere.

On the foreign exchange markets, the yen jumped to a record high against the dollar on expectations that companies, insurers and investors will repatriate more cash to help pay for the costs of the massive quake and tsunami that slammed the country's northeast on Friday.

The yen hit a high of around 76.30 per dollar in late New York trading, breaking the record of 79.75 in April 1995. It was trading at 79.50 yen per dollar during the Tokyo morning.