Dubai (CNN Business) OPEC is under pressure.

US crude oil prices have crashed almost 26% from their peak in October as fear of a shortfall in supply has turned into talk of a glut.

They suffered their worst day since September 2015 on Tuesday, plummeting 7% to a one-year low. That fall came just 24 hours after Saudi Arabia promised to pump fewer barrels in December and threw its weight behind a proposal for OPEC to cut production by one million barrels per day.

"Normally talk of Saudi production cuts would help to support the price of crude," said Russ Mould, investment director at AJ Bell. But other factors, such as soaring US production, are "exerting an even stronger influence," he added.

Another factor is President Donald Trump's decision to allow some countries to continue buying Iranian oil for six months following the reintroduction of US sanctions on November 5.

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