US oil ends down $2.12, or 3.1%, at $66.88 per barrel

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U.S. oil settled below $70 per barrel on Tuesday, giving up some of the gains seen on Monday when prices rallied for the first time in six sessions.

U.S. crude settled down $2.12, or 3.1 percent, at $66.88 per barrel, after jumping more than 4 percent Monday. Brent was last down about by about $2, or 2 percent, at $71, after gaining 3.4 percent.

Oil has plunged close to 40 percent in the past five months in its longest string of monthly losses since the 2008 crisis, as supply growth led by the U.S. shale oil boom exceeded demand. A decision by OPEC to maintain output has also pummelled prices.

"The market is trying to find some equilibrium,'' said Olivier Jakob, oil analyst at Petromatrix in Zug, Switzerland.

"There was a sharp drop last week and a rebound yesterday. Brent is trying to find out if it is going to be trading at the $60 to $70 range or $70 to $80.''

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Analysts said the market was going through a volatile adjustment phase that would lead to more erratic price movements before a more stable pricing environment was found.

"Saudi Arabia and OPEC no longer have the mechanism to balance markets from the supply side,'' said Mark Keenan, head of commodities research Asia at Societe Generale.

The bank cut its U.S. crude and Brent forecasts to an average of $65 and $70, respectively, for 2015 and 2016.

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Both Brent and U.S. crude touched five-year lows on Monday, with Brent dipping to $67.53 a barrel and WTI touching $63.72, before recovering to settle up on the day.

"Yesterday, much of the move higher right across the entire commodity complex ... suggests that there was a strong element of people increasing their allocation to commodities, taking advantage of these low prices,'' Keenan said.

Analysts say much lower oil prices may force some producers out of the market.

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As crude prices tumble, offshore drillers are increasingly considering "warm stacking'' their rigs to take them temporarily off the market, and they also threaten unconventional producers that have recently come to the market when prices were still higher.

New data suggest that a much-anticipated slowdown in the U.S. shale rig count has started arriving.