(BBC) - Oil prices have slipped after official US government data showed an unexpected rise in oil stocks last week.

The price of Brent crude dropped 1.2% to $56.24 a barrel, while US crude oil was down 0.4% after dipping below $50.

US crude oil stocks unexpectedly rose by 2.5 million barrels last week, according to Energy Information Administration data.

Analysts had expected a drop of 2.3 million barrels, and said the data had reinforced fears of oversupply.

"It's a very big surprise to see this type of a build [increase] at this time of year," said Tariq Zahir, founder at Tyche Capital Advisors.

The data comes after oil cartel the Organisation of the Petroleum Exporting Countries (Opec) reiterated again this week that it intended to maintain its current rate of oil output, a move aimed at maintaining its market share.

Previously, Opec has reacted to falling oil prices with a cut in output, a move that typically boosts the price.

The oil price has now fallen by almost half over the past year, with the price around $110 per barrel in June.

Production from North American shale companies has increased the supply of oil and gas, helping to depress prices.