New York's main contract, West Texas Intermediate (WTI) for delivery in November, finished down $1.59 from Friday's close to $77.61 a barrel, the lowest price since the end of September 2010.
Brent North Sea crude for November was down $1.05 to $101.71 in London midday trade.
"There is a selling in oil futures, tracking equities and the strengthening of the US dollar," said Victor Shum, senior principal of Purvin and Gertz energy consultants in Singapore.
"Global markets continue to be concerned about economic growth, especially in Europe and the US... Because of the concern about global markets, there is a flight to safety with investors flocking to the US dollar," he told AFP.
The European single currency slid to $1.3314, its lowest point since January, a head of a meeting of eurozone finance ministers to decide whether Greece should receive an eight-billion-euro loan installment.
Oil buying outside the United States often surges when the dollar, the key trading currency for oil, turns lower.
Austria's OMV said Monday it had received its first delivery of Libyan oil, 575,000 barrels of condensate, since production was halted in March due to the turmoil in the North African country.
"OMV's first cargo of Melittah condensate from Libya arrived last Friday at the port of Trieste," in Italy, the company said in a statement.
In Iraq, oil ministry spokesman Assem Jihad said crude output is now 2.9 million barrels per day (bpd) and on track to hit 3.0 million bpd by the end of the year
Meanwhile, last week's fire at Shell's large Singapore refinery in Pulau Bukom is likely to keep product prices firm for at least several weeks in the region, according to Priya Balchandani at Standard Chartered Bank.
"We expect extended support for refined oil products prices for up to a month...in light of this supply disruption," she said.
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