WASHINGTON (Reuters) - The top U.S. nuclear regulator told Congress on Wednesday the United States will not be hit by harmful radiation from Japan's crisis-hit reactors and that the evacuation area around them is smaller than what it would recommend.

Gregory Jaczko, head of the Nuclear Regulatory Commission, also said at a House energy and commerce subcommittee hearing that the spent fuel pool at Japan's troubled number four reactor has no water.

"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," Jaczko said.

When asked later by a lawmaker on the panel to further clarify his remarks, Jaczko said: "We believe at this point that unit four may have lost a significant inventory, if not lost all, of its water."

He added that there is also the possibility of a crack in the spent fuel pool in reactor three "which could lead to a lost of water in that pool."

While the NRC has 11 agency experts in Tokyo monitoring the situation, Jaczko stressed that the NRC's information "is limited" in what is happening at the nuclear power plant.

He said that radiation levels around the nuclear power plant may give emergency workers "lethal doses" of radiation, preventing them from getting near the plant.

"We believe that around the reactor site there are high levels of radiation," Jaczko said. "It would be very difficult for emergency workers to get near the reactors. The doses they could experience would potentially be lethal doses in a very short period of time."

It marks the first time the top U.S. nuclear regulator has testified before Congress on Japan's nuclear crisis.

(Editing by Alden Bentley)