Japan expressed confusion Thursday over an alarming finding by the U.S. nuclear regulatory body on the ongoing crisis at a nuclear power plant in Fukushima Prefecture and said Tokyo was somewhat slow in passing information along to Washington.

''There was a slight delay conveying to the U.S. side the information about whether or not there is water'' in a pool holding spent nuclear fuel rods at the quake-hit Fukushima No. 1 nuclear plant, Chief Cabinet Secretary Yukio Edano told a news conference.

His comment came in response to a senior U.S. nuclear official's remark Wednesday in Washington that there was no water left in the pool at the plant's No. 4 reactor, posing a risk of high-level radiation emission from the fuel rods exposed to the atmosphere.

Gregory Jaczko, chairman of the U.S. Nuclear Regulatory Commission, told a congressional hearing, ''There is no water in the spent fuel pool and we believe that radiation levels are extremely high, which could possibly impact the ability to take corrective measures.''

But a Ground Self-Defense Force chopper, which doused the overheating spent nuclear fuel pool with water Thursday morning at the No. 3 reactor, found that water is left in the pond at the No. 4 unit, according to the plant operator Tokyo Electric Power Co.

Jaczko also told Congress that spent fuel pools at the No. 2 and No. 3 reactors are also developing problems. The NRC has dispatched experts to Japan to provide Tokyo with assistance.

Based on the NRC's finding, the U.S. Embassy in Japan has asked American citizens living within an 80-kilometer radius of the Fukushima No. 1 power station to evacuate as a precautionary measure.

The Japanese government is currently setting the evacuation zone as areas within a 20-km radius of the plant and advises people outside the zone but within a 30-km radius to stay indoors.

Japan's Nuclear and Industrial Safety Agency said it believes the U.S. evacuation recommendation is ''not appropriate'' and will bolster information sharing with U.S. authorities so as not to cause misunderstandings.

Edano said that after the NRC chief made the remarks, the Japanese government provided U.S. experts with more detailed data.