Oil prices were narrowly mixed Thursday, with New York prices stuck below $50 after a rise in US crude stockpiles added to concerns over a supply glut, analysts said.

Brent North Sea crude for delivery in September fell 13 cents to stand at $56 a barrel in London midday deals.

US benchmark West Texas Intermediate (WTI) for September was up nine cents at $49.28 a barrel --  after closing Wednesday below $50 for the first time since April.

"There is little at the moment to suggest that prices will recover anytime soon," said Commerzbank analyst Carsten Fritsch.

"By expanding its production to a three-year high, OPEC is ensuring that the oil market remains amply supplied... OPEC is assuming that demand will pick up in the second half of the year," he added.

The US Department of Energy on Wednesday said the country's commercial crude stockpiles rose 2.5 million barrels last week, while supplies at the closely watched Cushing, Oklahoma, hub were up 800,000 barrels.

The report also showed US production staying at near-record levels of about 9.6 million barrels per day, bad news for a market already awash with crude from the Organization of Petroleum Exporting Countries (OPEC).

"US inventories expanded more than expected...  which exacerbated the oversupply problem, especially when Saudi is pumping at record production levels," said Bernard Aw, market strategist at IG Markets Singapore.

"OPEC produced 32.1 million barrels per day in June, with the largest increase coming from Iraq and Saudi Arabia. This was 2.1 million more than what the organisation said it will maintain," Aw added.

A strong US currency is putting further downward pressure on oil. With the commodity priced in dollars, it becomes more expensive for holders of weaker currencies, denting demand.

Expectations that the US central bank is on track to raise interest rates this year have boosted the dollar as investors flock to the currency in the hope of getting better returns.