Oil prices fell as much as 7 percent on Monday, dipping below $30 as investors' focus returned to the market's oversupply amid news that Iraq's output reached a record last month.
U.S. crude settled down $1.85, or 5.75 percent, at $30.34 a barrel, and later fell further to $29.71 per barrel.
Brent crude, the global benchmark, was last down $2.47 at $30.03 a barrel, losing about 6 percent from its closing price on Friday, when Brent surged 10 percent.
The biggest two-day rally since 2008 on Thursday and Friday helped put a stop to what analysts called an "irrational" sell-off that had sent oil prices crashing over 20 percent in January.
Brent hit a 12-year low of $27.10 on Jan. 20, before the two-day rally, while U.S. futures hit a 13-year low of $26.19, below the pivotal $30 levels.