China’s oil trade with Iran has stalled as Tehran withholds shipments and demands higher prices from its top client, tightening cheap supply for the world’s biggest crude importer, refinery and trade sources said.

The cutback in Iranian oil, which makes up some 10% of China’s crude imports and hit a record in October, could support global prices and squeeze profits at Chinese refiners.

The abrupt move, which one industry executive called a “default”, could also represent the backfiring of an October U.S. waiver on sanctions of Venezuelan oil, which diverted shipments from the South American producer to the U.S. and India, elevating prices for China as shipments dwindled.