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Donald Trump has criticised the Opec oil producers' group, saying crude prices are "artificially very high".

In a tweet, the US President said this was "no good" and "would not be accepted".

The main crude oil benchmark prices, Brent and West Texas Intermediate (WTI), fell around 1% after the tweet.

Oil prices have been rising over the past few months and on Thursday hit their highest levels since November 2014.

Opec and some other oil producing countries have been curbing production in an attempt to push up prices.

The curbs began at the start of 2017 and are currently set to expire at the end of 2018, although they could be extended when Opec members next meet in June.

In his tweet, Mr Trump said: "Looks like OPEC is at it again. With record amounts of Oil all over the place, including the fully loaded ships at sea, Oil prices are artificially Very High! No good and will not be accepted!"

President Trump's tweet rattled the market with both Brent crude and WTI falling by around 1%.

A boom in the production of oil from US shale fields, beginning at the start of the decade, increased global oil supplies and, by early 2016, prices had declined to below $30 per barrel.

In response, in December 2016, Opec's 14 member states and some non-Opec countries, such as Russia, agreed to rein in their oil production in the hope of boosting prices.

The supply controls largely worked and global oil prices have recovered - this week Brent crude hit three-year highs of almost $75 a barrel.

Mr Trump's attack on oil producers follows an attempt to reassert American power in global trade.

His administration has imposed tariffs on steel and aluminium imports, demanded changes to the North American Free Trade Agreement (Nafta) and withdrawn the US from the Trans-Pacific Partnership Agreement.

All policies were first announced by the US leader on social media.