Oil slipped to less than $66 a barrel on Monday, extending last week's decline of 8.4%, as investors focused on the halting economic recovery in the US.
Gains for the US dollar against the euro put pressure on oil, which had started the day higher on the back of Iran's test firing of missiles over the weekend, reported Reuters.
US crude for November delivery fell $0.43 to $65.59 a barrel by 06.43GMT (2.43am EDT), after having risen by as much as $0.64 in early trade. The contract settled up $0.13 at $66.02 on Friday.
London Brent crude fell $0.43 to $64.68 a barrel, according to Reuters.