The OPEC oil cartel has extended its self-imposed production cuts, but the market has not responded as intended.

Oil-traders reportedly believed that the nine-month extension of the cuts would be accompanied by tougher restrictions, but instead OPEC decided deeper cuts to prop up the price would not be part of the new deal.

After rising steadily in the lead-up to the OPEC meeting, Brent crude prices fell almost 5 per cent to $51.32 $US53.56 a barrel, while West Texas Intermediate went down 5.2 per cent to $48.71 a barrel.

The arrangement has seen OPEC nations throttle back on production by 1.4 million barrels per day, while 11 non-OPEC nations agreed to cuts of 400,000 b/d.

The deal has slowed the sharp the decline in oil prices, which have halved since 2014, and declined by 5 per cent since the start of the year.