By Timothy Puko and Georgi Kantchev 

Crude-oil prices fell Monday, the first day of trading after several OPEC members defended their refusal to take drastic action to prop up prices.

Oil officials from several leading members of the Organization of the Petroleum Exporting Countries on Sunday blamed producers outside of the group for the glut of oil on the market that has depressed prices. Oil prices have now fallen by nearly half since mid-June as U.S. shale producers battle Saudi Arabia and its Middle Eastern allies for market share.

Light, sweet crude for February delivery was down $1.45, or 2.5%, to $55.68 a barrel on The New York Mercantile Exchange after rallying earlier in the session by more than a percentage point. The February contract for Brent crude, the global benchmark, fell recently by $1.03, or 1.7%, to $60.35 a barrel on London's ICE Futures exchange, after flirting with $63 in morning trade.

"The reality of Saudi Arabia and (United Arab Emirates) saying, 'We're cool,' has taken over. It's still a bearish market," said Kyle Cooper, managing director of research at IAF Advisors, a Houston consulting firm. "It comes down to a battle between U.S. shale and Saudi Arabia."

Speaking at an energy conference in Abu Dhabi on Sunday, Saudi Arabia's oil minister, Ali al-Naimi, blamed a lack of coordination among non-OPEC producers, along with speculators and misleading information, for the price slump.

According to Mr. al-Naimi, who is OPEC's secretary-general and its most influential voice, the current market situation is temporary and prices will rebound.

Mr. al-Naimi's comments were echoed Monday by Iraq's oil minister, who said that OPEC remains united in its decision not to rein in production but to let the market decide the oil price. Prices have plummeted since OPEC decided on Nov. 27 to keep its production ceiling unchanged.

"The market doesn't have the momentum to sustain gains right now," said Kash Kamal, research analyst at Sucden Financial. According to Mr. Kamal, the comments by the officials provided some excuse for traders to enter long positions, but they quickly realized that fundamentals haven't changed. "The market is still under pressure, still oversupplied. It's business as usual."

Volatility in the oil market rose in December with the OVX index rising to its highest level since the inception of the series in 2012, the National Australia Bank said in a note. The OVX index measures the market's expectation of 30-day volatility of WTI crude prices.

Last week provided a practical primer on volatility with a pattern emerging in which early gains have been whittled away over the course of the day. On Thursday, crude had rallied by as much as 3% in early trade only to see the gains pared as negative sentiment overwhelmed the market.

Despite a rally on Friday, the price WTI crude fell 2.2% for the week, the fourth consecutive week of declines. During those four weeks WTI lost more than 26%, the largest percentage decline for that time period since the week ended Dec. 26, 2008.

Monday's early gains appeared to suffer the same fate, with an early rally sputtering by midday in Europe.

Nymex reformulated gasoline blendstock for January--the benchmark gasoline contract--was down 1.17 cents, or 0.8% at $1.5478 a gallon. January diesel lost 0.7 cent, or 0.4%, to $1.9552 a gallon.

Sarah Kent and Summer Said contributed to this article

Write to Georgi Kantchev at georgi.kantchev@wsj.com

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