Oil prices extended losses on Thursday after data showed US crude stockpiles gained unexpectedly, deepening market concerns on oversupply.

The Energy Information Administration (EIA) said in its weekly report on Wednesday that US oil inventories rose 1.7 million barrels last week to 521.1 million barrels, ending a 7-week streak of declines and beating market consensus of a 2.3-million barrels decrease.

Oilfield services company Baker Hughes said last week that the number of rigs operating in the US fields rose by 14 to 371 in the week ending July 22, the fourth weekly increase in a row.

Analysts said recent data have intensified fears on oversupply in the market, which weighed on oil prices. US crude prices hit three-month lows during the session on Thursday.

The West Texas Intermediate for September delivery lost 0.78 US dollars to settle at 41.14 US dollars a barrel on the New York Mercantile Exchange, while Brent crude for September delivery erased 0.77 US dollars to close at 42.70 US dollars a barrel on the London ICE Futures Exchange.