U.S. crude, or West Texas Intermediate, settled down 42 cents, at $ 43.46 a barrel, the lowest since March 2009.
Brent oil slipped almost 50 cents to $ 53.50 a barrel.
Brent last closed higher on March 11. Since then it has lost more than 7 percent, and traders believe it will break below $ 50.
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“OPEC members are still staunchly producing…and demand from Europe and China are really struggling to pick up,” said Kash Kamal, senior research analyst with Sucden Financial. “The overall tone is quite bearish.”
Kamal said any suggestion from the U.S. Federal Reserve, which will begin their two-day meeting later today, that they are moving toward their first interest rate rise in a decade could underpin crude prices, as it would suggest optimism from the Fed on the U.S. economy.
Others also see the possibility for markets to stabilise or even rise, with Vitol Chief Executive Ian Taylor telling a conference on Tuesday that oil markets would “come into balance” by the second half of the year.
However the potential of a nuclear deal that could end sanctions against Iran, allowing Tehran to send more of its oil into the market, also dragged on oil markets.
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Commerzbank said that if an agreement is reached, up to 1 million barrels per day of additional oil from Iran could reach the market in the second half of the year.
“When people look at the market now, they are looking at the possibility of an Iran resolution,” Bjarne Schieldrop, chief commodities analyst with SEB in Oslo.