Oil prices slipped into negative territory on Wednesday as a much larger-than-expected US crude stock build outweighed the threat to oil supply from potential military conflict between the US and Iran.

Brent futures were down 12 cents at $70.65 a barrel at 1607 GMT, while WTI futures were down 26 cents at $65.37. US crude inventories rose by 16 million barrels last week as refinery utilisation fell and imports increased, the Energy Information Administration said on Wednesday. That compared with analysts' expectations in a Reuters poll for a 1.5-million-barrel rise.