New York's main contract, West Texas Intermediate crude for delivery in May, was down 61 cents to USD 102.22 per barrel while Brent North Sea crude for June shed USD 1.13 to USD 120.08 in morning trade.
"Oil has slipped after 'constructive' talks between Iran and six of the world's biggest economic superpowers," said Justin Harper, market strategist at IG Markets Singapore in a note.
"This has helped take some of the 'war premium' out of energy prices," he added. Saturday's nuclear talks between officials from Iran and diplomats from the so-called P5+1 group comprising the United States, Russia, China, Britain, France and Germany have largely been described as "positive" by both sides.
However, Western officials have stressed that a great deal was expected of the Islamic republic at the next meeting in Baghdad on May 23.
The talks were aimed at persuading Tehran -- the second largest crude producer in OPEC -- to halt its controversial nuclear programme, which much of the international community believes is geared towards building an atomic weapon. Iran has so far insisted that it is enriching uranium for peaceful purposes including cancer treatment.