WASHINGTON (AP) _ President Clinton expressed deep concern today about a major leak at a uranium-processing plant in Japan today and pledged that the United States ``will do whatever we possibly can that will be helpful to them.''

``This is going to be a very hard day for the people of Japan,'' he said.

Clinton spoke after the accident sent radiation levels skyrocketing in the area around the plant in central Japan.

Undersecretary of State Thomas Pickering, speaking to reporters, said ``the situation is not trivial but is not a huge and galloping problem.'' He said Japanese officials are making a serious effort to bring it under control.

Clinton, meeting with reporters in the White House briefing room, said, ``We are all very concerned and our thoughts and prayers are with the people in Japan today because of the uranium plant reaction.

``You can only imagine how difficult this must be for them, quite apart from whatever the facts are,'' he said, apparently referring to Japan's sensitivity over nuclear issues after suffering through two atomic bomb blasts in World War II.

``We are doing our best to determine what in fact has happened and what assistance we can give,'' Clinton said. ``We will do whatever we possibly can that will be helpful to them. And we will try to be as comprehensive and prompt about it as possible.''