"Our main problem is the compliance. In the next meeting ... this will be the main subject to be raised," he told reporters.

"I'm not worried about (softening) demand, I'm worried about the quotas ... OPEC should be more committed to their quotas. There is a bit of slippage here and there," he added.

The Organization of the Petroleum Exporting Countries (OPEC) meets in Vienna on Oct. 14 and Libya, Iraq, Qatar, Ecuador and Kuwait have all said they see no need for a change in policy.

Compliance among members with implied output curbs has sunk to just above 50 percent as oil has largely traded within the OPEC's preferred zone of $70 to $80 per barrel.

OPEC would be comfortable with 75 percent to 85 percent compliance, not the current level of 52 percent, he said.

Crude oil prices were likely to be between $75 and $80 per barrel in the first quarter of 2011, Sheikh Ahmad said — broadly unchanged from current levels.

Venezuela's Oil Minister Rafael Ramirez said on Sunday he was comfortable with global oil prices, but OPEC should focus on high global crude inventories.

The organization has kept its output targets unchanged for almost two years, though it has boosted supply informally since 2009 as oil prices have largely traded in the $70-$80 a barrel range seen by some members as acceptable.