The price of benchmark crude oil has fallen almost four percent as traders question whether Opec can agree on a deal to limit production.
In September, the oil exporting cartel members voted for the first production cut in eight years.
Details of the agreement are due to be finalised at a formal Opec meeting in Vienna on Wednesday.
But key Opec members appear to disagree over the plan and some analysts believe the meeting may not produce a deal.
Indonesian Energy Minister Ignasius Jonan said he was not sure Opec would manage to forge an agreement: “I don’t know. Let’s see. The feeling today is mixed.”
Brent crude oil was down $1.76 per barrel at $46.48, and US crude was down $1.80 at $45.28.
Analysts at Barclays said: “Volatility is set to be high in the oil market in the days ahead.”
In-depth negotiations will be needed on Wednesday to cement a deal, Goldman Sachs analysts said.
In September, Opec pledged to limit production by about 700,000 barrels a day, although Iran was being allowed to increase production.
Disagreements between Iran and its regional rival, Saudi Arabia, had blocked earlier attempts to reach a deal.
Many of Opec’s smaller members wanted limits after oil prices fell from $110 a barrel over the past two years after oversupply and slowing demand.