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New York's main contract, light sweet crude for delivery in June, lost 91 cents to $111.37 a barrel, while Brent North Sea crude for June dipped 66 cents to $123.00.
"Traders and investors are now simply booking in profits after returning from a long weekend," said Victor Shum, a Singapore-based analyst with the Purvin and Gertz energy consultancy.
But he added that the continuing unrest in the oil-producing Arab world would sustain high oil prices.
"The unrest in the Middle East and North Africa region continues to be a key factor supporting high oil prices, especially the protests in Yemen and Syria besides the ongoing unrest in Libya," Shum told AFP.
Yemen's opposition said Monday it had agreed to a plan under which President Ali Abdullah Saleh would step down after 30 days, as two protesters were shot dead amid continued demonstrations demanding his ouster.
In Syria, government troops backed by tanks rolled into the flashpoint town of Daraa on Monday, killing at least 25 people, witnesses said.
And in Libya, rebels said they had pushed Moamer Kadhafi's troops out of the besieged city of Misrata, after his Tripoli compound was attacked in a NATO air strike.
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