A Russian firefighting plane went missing Friday while dousing a forest blaze in Siberia, Interfax news agency reported.
The Il-76 jet was operated by Russia's Emergencies Ministry. It had dumped water on a forest fire in Kachugsky Region near the city of Irkutsk and was returning to base when radio contact was lost.
Interfax quoted one official as saying the smoke was so thick visibility was "close to zero."
The cargo plane, modified to carry water, had a crew of up to 10 people aboard, Interfax said.
Investigators were quoted saying they do not know where the plane came down. Much of eastern Siberia is uninhabited wilderness.
It was the plane's second flight of the day to drop water on the blaze, Interfax said, citing a source.