Benchmark prices climbed as much as 2% on the London Metal Exchange on Tuesday to $12,159.50. Copper is piling up in American warehouses as the potential imposition of new levies prompts buyers to secure shipments and has opened up an arbitration trade. “The US is still in stock-building mode, and I would expect that to continue until we get further information from the US government,” said Helen Amos, a commodities analyst at BMO Capital Markets Ltd.