Mining giant Rio Tinto's exports of iron ore have been disrupted after a fire engulfed a section of one of its ports on the coast of Western Australia's minerals-rich Pilbara region.

The Anglo-Australian miner on Monday revealed it had declared "force majeure" on its contracts with affected customers as a result of last week's blaze.

The fire, on January 10, broke out at Rio Tinto's Cape Lambert Port near Karratha, causing damage to a part of the plant that separates its Robe Valley lump and fine products, a company spokesman said.

"As a result, Rio Tinto has declared force majeure on impacted customers," the spokesman said.