Oil falls $3 to 4-year low after OPEC leaves output

If OPEC doesn't cut production, oil will fall further: Pro

Brent crude oil fell more than $3 to a fresh four-year low under $75 a barrel on Thursday after OPEC decided not to cut production, despite a huge oversupply in world markets.

Asked whether the oil producer group had decided not to reduce production, Saudi Arabian Oil Minister Ali al-Naimi told reporters: "That is right."

Read MoreOPEC will not cut oil production: Saudi minister

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Oil prices have fallen by more than a third since June as increasing production in North America from shale oil has overwhelmed demand at a time of sluggish global economic growth.

Ministers from the Organization of the Petroleum Exporting Countries had been discussing whether to agree a production cut at their meeting in Vienna.

Read MorePre-OPEC Saudi, Russia oil meet fails to agree output cut

Benchmark Brent futures were down by $2.96 a barrel at $74.79 by 1520 GMT, after hitting a low of $74.36, their lowest since August 2010.

U.S. crude was at $71.07, down $2.62.

"Oil prices are now completely in the hands of the market," Dominic Chirichella, director of New York-based Energy Management Institute, told Reuters Global Oil Forum.

Kuwaiti Oil Minister Ali Saleh al-Omair said there would be "no change" to OPEC's existing oil production target following the meeting.

Read MoreEurope higher; oil below $75 as OPEC leaves output

The cartel will meet again in June next year, said an OPEC delegate. Ehsan Ul-Haq, senior oil market consultant at KBC Energy Economics, in Vienna for the OPEC meeting, said a KBC report expected oil prices stay under $80 a barrel.

"The probability of oil prices going below $70 a barrel is 20 percent, remaining in a range of $70-80 a barrel is 40 percent," he said.