Oil prices plummeted after Greece rejected debt bailout terms.
In a Sunday referendum, more than 61 percent of Greeks voted " NO", rejecting a proposal from international creditors that included pension cuts, tax increases and other measures on Monday, Xinhua reported.
Analysts said the "NO" vote increases the likelihood that Greece may eventually exit the Eurozone and add uncertainty of European economy.
Finance ministers from euro zone countries are expecting new proposals from Greece in talks held Tuesday, Eurogroup said in a statement on Monday.
The Greek debt crisis boosted the dollar. A stronger greenback made the dollar-priced crude more expensive and less attractive for buyers holding other currencies.
Also, China's stock markets have declined in recent weeks, which sparked worries among investors about oil demand from the world's important consumer.
Traders were also eyeing the progress of Iran nuclear talks. The negotiations between world powers and oil producer Iran to curb the latter's nuclear ambitions dragged into final days.
The West Texas Intermediate for August delivery moved down $4.4 to settle at $52.53 a barrel on the New York Mercantile Exchange, while Brent crude for August delivery decreased $3.78 to close at $56.54 a barrel on the London ICE Future Exchange.
