Brent crude dropped more than $3 on Monday to below $106 a barrel, while U.S. oil fell more than a dollar to below $82, on the potential for a resumption of exports from OPEC-member Libya as a six-month civil war there appeared close to an end.
A bleak global economic outlook has also dampened prices, analysts said.
Libya pumped around 1.6 million barrels per day (bpd), nearly 2 percent of global supply, before the war cut output. Most of Libya's high-quality crude flowed to European refiners, and tightening supply after Libyan exports stopped drove Brent to a two-year high of $127.02 in April.
Brent crude dropped $3.47 to $105.15 per barrel at 3:31 a.m. EDT. U.S. crude fell 24 cents to $82.02 a barrel after dropping to as low as $81.13 earlier. The front-month September U.S. crude contract expires on Monday.
It may take years for output from Libya, the world's 17th-largest oil producer, to fully recover, but rebels hope to resume oil output -- Libya's main revenue earner. Analysts say output of as much as a million bpd could be feasible within months.
Rebels swept into the heart of Tripoli and crowds took to the streets to celebrate what they saw as the end of Muammar Gaddafi's four decades of power, but a government fightback was reported as dawn broke on Monday.
You will see a relaxation in the supply of crude to the region as a result of what is happening in Libya, said Jonathan Barratt, managing director at Commodity Broking Services in Sydney on how supplies coming back are hitting prices.
Tight supplies of Libya's light sweet crude in Europe helped fuel a widening of the spread between Brent and U.S. WTI crude. The spread is already narrowing and could contract further with the prospect of a resumption in Libyan supplies, Barratt said.
The important thing to note is that Brent and WTI (spreads) should be trading at $1-$2. The spreads between WTI and Brent should continue to unwind as Libya's light sweet crude is added back to the market.
Concern about the health of global markets kept investors skittish on Monday.
The longer term view is that WTI is going to fall off, said Tony Nunan, a risk manager with Mitsubishi Corp in Japan.
The total market looks pretty weak because of what has happened in the last couple of weeks, with economic data now clearly showing that the U.S. and Europe are slowing down.
European stocks extended four weeks of losses on Monday, tracking jittery Asian shares lower, while gold shot to new highs as investors worried about the sluggish U.S. economic outlook and Europe's festering debt crisis.
Market players are awaiting a speech from the U.S. Federal Reserve Chairman Ben Bernanke on Friday at a lodge in Wyoming's Jackson Hole, where policymakers and academics meet once a year to talk shop.
Bernanke just did what he could to keep interest rates low until 2013. It will be tough to follow that up with a third round of quantitative easing. That could come as a disappointment and could be another bearish element to the market, said Nunan.
Last year, Bernanke used the podium to suggest the Fed could help growth by buying long-term bonds, a prelude to a program enacted soon afterward that did just that.
No grand new plan is expected to be hatched at this year's meeting, but investors are concerned the U.S. may again fall into recession and will be watching closely for any sign of a downgrade in Bernanke's economic outlook.
(Additional reporting by Cho Mee-Young in SEOUL and Randy Fabi in Singapore; Editing by Himani Sarkar)