Oil prices fell about 3 percent to seven-month lows on Tuesday after increases in supply by several key producers overshadowed high compliance by OPEC and non-OPEC oil producers with a deal to cut global output.
Benchmark Brent fell as low as $45.42, its weakest since Nov. 15, just before OPEC and other producers agreed to cut output by 1.8 million barrels per day (bpd). It was down 96 cents, or 2.1 percent, to $45.95 a barrel 2:37 p.m. (1837 GMT)
U.S. West Texas Intermediate crude oil contract for July fell to a session low of $42.75 a barrel, the weakest level since Nov. 14. It ended Tuesday's session down 97 cents, or 2.2 percent, at $43.23 on its final day as the front-month contract.
The closing level, the lowest since Sept. 16, was more than 20 percent below WTI's 52-week closing high in February, putting the commodity in bear market territory.
WTI's August contract, which accounted for most of the volume in U.S. crude futures on Tuesday, was down 95 cents, or 2.1 percent, at $43.48.
Both benchmarks are down about 15 percent since late May, when OPEC, Russia and other producers extended their limits on production until the end of March 2018.
"Given the expectation that you'll see higher production levels in several areas of the world, it's going to offset all they're taking off the market," said Gene McGillian, manager of market research at Tradition Energy.
OPEC and non-OPEC oil producers' compliance with the deal to cut output reached its highest in May since they agreed on the curbs last year, reaching 106 percent last month, a source familiar with the matter said.
OPEC supplies, however, jumped in May as output recovered in Libya and Nigeria, both exempt from the production reduction agreement.
Libya's oil production rose more than 50,000 bpd to 885,000 bpd after the state oil company settled a dispute with Germany's Wintershall, a Libyan source told Reuters.
Nigerian oil supply is also rising. Exports of Nigeria's Bonny Light crude are set to reach 226,000 bpd in August, up from 164,000 bpd in July, loading programs show.
"The increasing August export program in Nigeria and the jump in Libyan oil output should pressure oil prices further in the short term," said Tamas Varga, senior analyst at London brokerage PVM Oil Associates.
"If we get bearish U.S. oil statistics this week, we could see a test of $45 on Brent," Varga said.
Ahead of weekly U.S. inventory reports, U.S. crude oil stocks were forecast to have fallen for the second straight week, while gasoline supplies were seen unchanged after last week's data showed an unexpected build that weighed down the market.
Industry group American Petroleum Institute (API) is scheduled to release its data for last week at 4:30 p.m., and official government report is due at 10:30 a.m. EDT on Wednesday.
"Recent data points are not encouraging," Morgan Stanley analysts said in a note to clients. "Identifiable oil inventories - both crude and product in the OECD, China and selected other non-OECD countries - increased at a rate of (about) 1 (million bpd) in Q1."
Hedge fund managers have become very bearish about the outlook for oil prices as production from countries outside OPEC grows and threatens to undermine the effectiveness of OPECs output controls.
The $1.3 billion Andurand Commodities fund was down 17.33 percent for the year by May 31, according to an HSBC report. Merchant Commodity Fund was down 12.38 percent by June 9, the report said.
"At the moment sentiment is bearish and traders seem happy to keep selling into every rally," said Fawad Razaqzada, financial markets technical analyst at Forex.com.
— CNBC's Tom DiChristopher contributed to this story.