International benchmark Brent plumbed two-week lows for the third straight session as President Donald Trump renewed pressure on OPEC to support the market.
As of 13:50 GMT, Brent fell to $65.75 a barrel, with the lowest since February 15 at $65.24.
Brent lost 0.1% on Friday, the second loss in a row on profit-taking.
Oil added 1.5% last week, the second weekly profit in a row on cuts by OPEC and Russia.
In a tweet, Trump warned that oil prices are rising way too fast, urging OPEC to relax and allow prices to settle down.
OPEC is implementing a deal with Russia to cut global output by 1.2 million bpd.