Oil prices tumbled nearly 4 percent on Monday on concern that record Iraqi crude exports and rising U.S. output would undermine OPEC's efforts to curb global oversupply.
U.S. crude futures settled at $51.96 per barrel, down $2.03, or 3.8 percent. That was the weakest daily closing level since Dec. 16.
Brent crude futures were down $2.22, or 3.9 percent, at $54.88 a barrel at 2:35 p.m. ET (1912 GMT), near the lows of the day.
In Iraq, OPEC's second-biggest producer, oil exports from the southern Basra ports reached a record high of 3.51 million barrels per day (bpd) in December, the oil ministry said.
Iraq's oil ministry underscored that the high levels from the south would not affect the country's decision to lower production in January to comply with the OPEC agreement. But some remained concerned over the feasibility of the cuts — which would have to come from the north.