Light, sweet crude for October delivery fell $3.08 to settle at $103.26 on the New York Mercantile Exchange, the lowest settlement price since April 1. The contract rose 11 cents to settle at $106.34 in volatile trading Monday.
In aftermarket trading Tuesday, prices tumbled more than $4 a barrel to a new five-month low of $101.74.
Crude's decline puts the contract within striking distance of the psychologically important $100 threshold, a level first reached Feb. 19.
In London, October Brent crude fell as low as $98.94 a barrel on the ICE Futures exchange, slipping below $100 for the first time since April 2. The contract later settled $3.10 lower at $100.34 a barrel.
"It looks like a total exodus out of the market right now," said Phil Flynn, an analyst at Alaron Trading Corp. in Chicago.
Oil market traders were also keeping a close watch on an OPEC meeting in Vienna, Austria.
Early today, OPEC oil ministers decided to curb overproduction by more than 500,000 barrels.
The move is a compromise meant to avoid new turmoil in oil markets while reflecting OPEC attempts to prevent prices from falling too far.
An OPEC statement issued after oil ministers ended their meeting said the organization agreed to produce 28.8 million barrels a day. OPEC President Chakib Khelil said that quota in effect meant that member countries agreed to cut back 530,000 barrels a day in overproduction.