Oil prices in New York have tumbled to their lowest level since mid-March helped by unplanned refinery shutdowns in the US midwest.
The benchmark West Texas Intermediate for September delivery fell US$2.10 to US$98.17 a barrel on the New York Mercantile Exchange.
In London, European benchmark Brent oil for Septemberdelivery declined US49c to US$106.02 a barrel.
Andy Lipow, head of Houston consultancy Lipow Oil Associates, said news that a number of midwest oil refineries were experiencing outages put a "significant amount of pressure on the WTI".
The concern is that crude inventories will build at the closely-watched Cushing, Oklahoma oil-trading hub where WTI is priced.
Key questions include the effect of new sanctions on Russian oil production and the impact of continued violence in Libya on the North African country's output.
Eurasia Group predicted Brent would continue to trade in the US$105-US$110 a barrel range in the third quarter.
