By early afternoon in Europe, benchmark crude for January delivery was down 99 cents to $98.42 a barrel in electronic trading on the New York Mercantile Exchange. The contract rose $1.07 to settle at $99.41 on Friday.

In London, Brent crude fell 74 cents at $107.88 on the ICE Futures exchange.

Measures agreed to at last week's summit of EU leaders in Brussels meant to tackle the debt crisis sweeping across the continent was initially welcomed by investors, but many details remained unclear and analysts feared that future steps — like a central authority to supervise national budgets — did not really offer solutions to current problems.

The Nymex contract "continues to trade below the psychological level of $100 a barrel ... as persistent concerns about the eurozone's economic stability weighed heavily on market sentiment," said a report from Sucden Financial in London. "It seems that risk appetite is off the table (Monday), as investors try to digest the news while the global macroeconomic picture continues to remain fairly gloomy."

The Organization of Petroleum Exporting Countries meets Wednesday in Vienna, and analysts will be closely watching whether the 12-nation group can agree on production policy. At OPEC's last meeting in June, members failed to reach consensus and output quotas were left unchanged.

However, the International Energy Agency estimates OPEC produces about 10 percent more than its official quotas. Cheating on quotas and Middle East upheavals and rivalries threaten to crack OPEC unity, which could lead to more crude on the market, analysts said.

"With economic sentiment on the downturn and Libyan output having been ramped up faster than expected, calls for a cut in output targets have been voiced, most notably from Iran," said analysts at JBC Energy in Vienna. "However, we do not expect any official output change given the persistent geopolitical risks and supply uncertainties in the market."

"In our view, oil prices are more likely to be confronted with substantial upside- rather than downside risks given the considerable supply tightness in the market," JBC Energy said.

Crude has jumped from $75 in October after the U.S. economy has shown signs of better than expected economic growth in recent months. Analysts expect global economy growth to slow next year, which would likely undermine crude demand.

In other energy trading on the Nymex, natural gas dropped 7.7 cents to $3.24 per 1,000 cubic feet. Heating oil fell 1.15 cents to $2.9010 a gallon and gasoline futures slid 0.01 cent to $2.596 a gallon.

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Alex Kennedy in Singapore contributed to this report.