Oil prices have fallen to their lowest level this year in a move attributed to traders' impatience as they waited for Opec to finalise details of an output cut.
The market is also anticipating another rise in already hefty US crude inventories.
New York prices at one stage hit $57.24 per barrel, a low point last reached on December 19th 2005. London's Brent crude oil, meanwhile, fell 35 cents lower at $58.30.
Opec ministers have agreed oil markets are oversupplied by around one million barrels per day, but they have yet to decide on whether to make a cut from a notional 28 million-bpd production ceiling or from actual supply of nearly 27.5 million bpd in September.
They are also divided over whether to hold an emergency meeting to formalise what would be Opec's first output cut since 2004.
Some analysts say prices could fall further with or without an output cut, as lower Opec production would increase spare capacity.
There are also concerns about the health of the US economy, which could lead to reduced demand.