Oil futures tumbled 3 percent on Monday after US President Donald Trump called for OPEC to "relax and take it easy" on boosting crude prices, which he said were climbing too high.
Brent crude oil futures were down $2.03 at $65.09 a barrel by 11:20 a.m. EST (1620 GMT). West Texas Intermediate (WTI) crude futures fell $1.73 to $55.53 a barrel.
"Oil prices getting too high. OPEC, please relax and take it easy. World cannot take a price hike - fragile!" Trump tweeted, his latest in a series of tweets or comments made regarding oil prices since April 2018.
After the tweet, prices reversed earlier gain that had built on expectations for tightening supply and hopes for an agreement a day after the president promised progress in coming weeks over US-China trade talks.
Crude prices have risen by about 20 percent since the start of the year when the Organization of the Petroleum Exporting Countries and non-member producers, such as Russia, cut production to reduce global supply.
US sanctions on exports of crude from Iran and Venezuela have also helped tighten the market and support prices as production in the United States surges.
"If you read into it, I think there's speculation there will, in fact, be another round of waivers granted to countries and companies to buy Iranian oil," said John Kilduff, a partner at Again Capital Management, said about Trump's tweet. "That's also why you're seeing the negative reaction."
Washington surprised oil markets after granting waivers to eight Iranian oil buyers when the sanctions on oil imports started in November. Brent futures fell 22 percent that month and the waivers influenced OPEC's decision to agree in December to supply cuts starting in 2019.
Adding to the uncertain supply picture was political unrest in both in Venezuela and Libya.
"Supply risk is ever present with Venezuelan tensions brewing a notch higher ... the National Oil Corporation in Libya refusing to start production at the El Sharara field," Harry Tchilinguirian, global oil strategist at BNP Paribas in London, told the Reuters Global Oil Forum.
Nigeria, Africa's largest oil exporter, where as many as 39 people were killed in election violence over the weekend, also added production risk.Goldman Sachs analysts said on Monday that "the near-term outlook for oil is modestly bullish over the next two to three months", but added that the outlook for later in 2019 was weaker due to surging U.S. exports and an "an increasingly uncertain economic, policy and geopolitical backdrop".