World oil prices fell sharply after OPEC maintained its output ceiling even as it expressed worry over the impact of weak global and eurozone growth on demand for crude.
In New York, WTI light sweet crude for delivery in July lost $1.64 to finish at $91.97 a barrel, under the $92 level for the first time since the beginning of May.
In London, Brent North Sea crude for July gave up $1.80 at $100.39 a barrel.
The Vienna meeting of the Organization of Petroleum Exporting Countries, which pumps about a third of global oil supplies, as expected held the production ceiling of 30 million barrels a day, where it has stood since late 2011.
But the group's confidence in its ability to hold prices where they are at that level of production was not clear.
"Although not unexpected, the combination of the retention of the same target, and recent increases in actual production to 30.5 million barrels a day that calls compliance into question, suggest risk of an ongoing surplus," said Timothy Evans at Citi Futures.
Pointing to a key market threat, in a statement OPEC said that "downside risks to the global economy, especially in the OECD region, remain unchecked."
"Everybody has agreed to maintain the level of production and we are monitoring the market," Venezuelan Energy Minister Rafael Ramirez told reporters.
"We are monitoring the market because the economic situation in the EU is difficult, and when we are meeting next time in December we will have more elements," Ramirez said.
"We have to defend the price, we're going to defend the price," he added.
The group made note of the impact on the global market of the rising production in North America due to oil and gas extracted from shale and Canada's tar sands oil.
"We will follow it up as with any sort of energy," OPEC secretary-general Abdullah El-Badri told a post-meeting press conference.
"We have to look at accurate information and see how much this type of oil will be sustainable into the future."
Despite the apparent focus on demand growth, scheduling their next output conference in December suggested a measure of confidence.