US commercial stockpiles of crude oil last week dropped to their lowest level since 1976, leaving the world's largest energy consumer increasingly vulnerable in the run-up to a possible war with Iraq.

The American Petroleum Institute, the leading industry research group, revealed on Thursday that crude stocks had fallen to 268.3m barrels, shaving nearly 50m barrels off levels a year ago. At the same time heating oil supplies have shrunk more than 20 per cent since last year, amid the coldest winter the US north-east has experienced in recent years.