US President Donald Trump has urged Saudi Arabia to sharply increase its oil production to combat the rising cost of fuel.

Mr Trump tweeted that he had asked Saudi ruler King Salman to raise oil output by up to two million barrels a day.

"Prices to [sic] high! He has agreed!" the president added.

Mr Trump said the move was needed due to "turmoil and dysfunction in Iran and Venezuela".

Oil prices rose last week, partly due to US plans to reimpose sanctions on Iran, a major oil producer.

The Opec oil producers' group agreed to increase output, as did Russia, but this failed to reassure markets.

The Saudi Press Agency confirmed that President Trump and King Salman had spoken by phone, giving few details. It said they had discussed the need to "preserve the stability of the oil market".

The statement did not confirm that Saudi Arabia had agreed to the two million barrels a day figure.

Saudi Arabia is the world's biggest exporter of oil and produced about 10 million barrels a day in May. It is reported to have between 1.5 million and two million barrels a day of spare capacity - but experts told The Wall Street Journal it might not be keen to meet the president's request.

"Saudi Arabia does not really like going beyond 11 million barrels a day and has no intention of expanding its current production capacity. It is expensive," a Saudi official told the paper.

Mr Trump has repeatedly criticised Opec even though US ally Saudi Arabia is a core member.

On 20 April he tweeted that oil prices were "artificially very high", saying this was "no good" and "will not be accepted!"

Iran, another Opec member, has accused Mr Trump of trying to politicise the group and has blamed Riyadh for doing his bidding.

On Saturday, Iranian Supreme Leader Ayatollah Ali Khamenei said the US was trying to drive a wedge between Iranians and their government using "economic pressure".

"Six US presidents before him tried this and had to give up," Mr Khamenei cautioned on his website.

The value of Iran's currency, the rial, has tumbled since Washington backed out of the Iran nuclear deal in May.

Earlier this week, thousands of traders at Tehran's Grand Bazaar marched in protest against rising prices and the plummeting value of the rial. It was the biggest protest the city has seen since 2012. (BBC)

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