Iron ore's tumble into the $US30s threatens the world's biggest miners as prices approach break-even costs, according to Capital Economics.

The most expensive operations at the four largest suppliers are on the verge of making losses at rates below $US40 a metric ton, said John Kovacs, senior commodities economist at Capital Economics in London, who estimates their break-even levels at $US28 to $US39, taking into account freight and other costs. While these producers will keep output strong, they'll be constrained by low prices, he said by email on Monday.

Bloomberg