Two reactors were shut off early Friday at the Chernobyl nuclear power plant because a "mistake by personnel" caused a problem in their water-cooling system, the state news agency Tass said.

"This is purely a technical stop, which poses no danger for people and the natural environment," shift supervisor N. Fedoseyev was quoted as saying.An explosion and fire at the plant in 1986 sent a radioactive cloud across neighboring European countries and around the world, causing irreparable damage to the agriculture and ecology of the immediate region.

Official reports say 31 people were killed in the partial meltdown, but unofficial reports say 250 died, and four years after the accident more than 2 million people are still living on contaminated lands in the region.

The shutdown Friday came at about 1 a.m. (5 p.m. EST Thursday) and was reported Friday night - in contrast to 1986, when the government waited days before releasing the information.

Chernobyl's first and second power units, or reactors, were shut down after levels of high-quality cooling water dipped below permissible levels, Fedoseyev was quoted as saying.

The third working reactor continued to operate, he said.