Benchmark West Texas Intermediate crude lost $2.26, or 2.6 percent, to finish at $85.70 per barrel on the New York Mercantile Exchange. In London, Brent crude fell $3.098, or 2.7 percent, to end at $109.14.
The dollar rose against other major currencies, including the euro. That tends to push down oil prices because oil is priced in U.S. currency and a rising dollar makes crude more expensive for investors with foreign money.
Oil has been falling since Friday, when European leaders delayed more emergency funds to Greece. If the Greek government defaults, it could destabilize the economies of other countries as well.
Greece's finance minister said Monday that his country will try to avoid international "blackmail and humiliation" by speeding up reforms and civil-service staff cuts.
Analyst Stephen Schork said energy traders are looking for greater assurances that the eurozone leaders, especially Germany, remain committed to bankrolling aid programs for weaker members.
"The northern European countries are tired of paying for their neighbors on the Mediterranean," he said.
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