WASHINGTON  - A malfunction in a steam pipeline has forced officials at the Chernobyl nuclear power plant in Ukraine to cut power back 50 percent, even as forest fires spread the remnants of radiation from a 1986 disaster at the plant, said a U.S. official Wednesday.

The new malfunction caused the turbo generator in the reactor, the only one in operation, to switch off. Repairs are expected to take until Saturday to complete, the official said.

There is no evidence of radiation as a result of the malfunction, the official told The Associated Press.

But, at the same time, the official said, forest fires in the area had caused the circulation into the air of remnants of radiation in roots and stems of plants, with the result that the radiation level in Kiev was elevated slightly,

The official, speaking on condition of anonymity, called the malfunction a glitch that disabled the sixth turbo generator in the reactor, the only one still functioning in Chernobyl. As a result, the reactor was powered down to by about 50 percent, the official said.

President Clinton is due to visit Kiev June 6 after summit talks in Moscow with Russian President Vladimir Putin.

A White House official said there were no radiation concerns at this point.

The official, speaking on condition of anonymity, said there was no reason for Clinton to change plans.

It's not a big crisis, the official said, adding: It does not appear to be serious.