At least 10 heavily armed men stormed the runway at Carajas airport, 1,300 miles northwest of Rio de Janeiro, as the gold was being loaded from a helicopter to the plane.

The robbers overpowered guards in a shootout, and took the pilot, co-pilot and two security guards hostage.

Malu Molter, spokeswoman for the mining company Cia Vale do Rio Doce, said nobody was injured.

Police have few clues about where the robbers are now

The hijackers made the pilot fly to the remote Amazon jungle town Sao Felix do Xingu, 125 miles to the southwest.

There the men transferred the gold to another plane and freed the hostages before taking off again.

Ms Molter said the gold had been mined at the company's Igarape da Bahia gold mine, but she did not know where the plane was headed before it was hijacked.

A police official in Carajas said police had no good leads as to where the robbers had gone.

The remote jungle area is not covered by radar so the authorities had no way of monitoring the plane's path. It could have landed at any of hundreds of landing strips in the area.

Cia Vale do Rio Doce mines gold and the world's largest iron ore deposit in Carajas.

The mineral-rich region has long been a magnet for Brazil's gold diggers.