New York's main contract, light sweet crude for July delivery, lost 95 cents to USD 99.15 a barrel, while Brent North Sea crude for July delivery dipped 70 cents to USD 111.69.
"Crude oil prices were weaker as US economic recovery is sputtering and fuel demand may drop," said Ker Chung Yang, commodity analyst from Phillip Futures in Singapore.
The United States is the world's biggest oil consumer.
Traders are looking to the Organisation of the Petroleum Exporting Countries' (OPEC) bi-annual ministerial meeting scheduled to be held in Vienna on June 8.
"It is coming close to the OPEC meeting held first week of June, 2011, which will be closely watched by the market," Ker told AFP.
OPEC pumps 40 per cent of the world's crude and any comments by OPEC have an impact on oil prices.
While weaker US macroeconomic data weighed on prices, "the underlying fundamentals in the oil market itself remain very robust," Barclays Capital analysts said in a commentary.