The world’s biggest miners were on Monday night locked in tense talks with Chinese steelmakers over a new iron ore supply deal, with Rio Tinto threatening to sell the ore into spot markets should no agreement be struck on Tuesday.

Spot iron ore prices on Monday rose to their highest in four months, above the annual benchmark level at which deals have been struck with big steelmakers elsewhere in Asia. The increase in prices added a fresh complication to the fraught negotiations.