Oil fell more than $2 a barrel to its lowest in more than a year as investor concerns over conflict in Ukraine and Iraq eased, and as higher Libyan output added to already ample supplies.
Crude jumped on Friday after the government in Kiev said its artillery had partially destroyed a Russian-armoured column. While fighting between Ukrainian forces and pro-Moscow separatists continues, fears of a further escalation proved unfounded.
"It became apparent on Friday just how nervously the market reacts to geopolitical risks. Right now, the market is just giving up all the gains it made after this news," said Eugen Weinberg, analyst at Commerzbank.
Brent crude was down $2.14 at $101.39 a barrel by 1530 GMT,its lowest level since June 2013, after rising $1.52 on Friday. US crude fell $1.34 to $96.01, after a gain of $1.77 in the previous session.
In Iraq, Kurdish peshmerga fighters and Iraqi counter-terrorism forces have pushed Islamic State militants out of Mosul dam, state television reported on Monday, while higher Libyan output threatens to compound ample supply.
"Re-opening of the biggest oil ports in the east could raise Libyan oil supplies further in the next few weeks, adding to the existing oversupply on the European market," Commerzbank said in a report. "These developments make an imminent Brent recovery unlikely."
Advances by militants in Iraq in June prompted a rise in oil prices, although the fighting has yet to affect oil supplies from southern oil ports, the outlet by which almost all of Iraq's crude reaches world markets.
Libya's production, disrupted for months by strikes and protests, had risen to 535,000 barrels a day (bpd) on Sunday, a spokesman for the state oil company said, higher than previously reported. Still, production is still far below the 1.4 million bpd it pumped last year.
Adding to easing tensions and rising output in Libya, Brent has been weighed by low demand from refineries in Europe and Asia, pushing prices into a deepening contango structure where immediate supply is cheaper than oil for later delivery.
Nonetheless, another flare-up in geopolitical tensions or a reversal of Libya's output gains could send prices back up, especially as seasonally higher demand approaches, analysts say.
"Sure, supplies are plentiful, but short-term output glitches in Libya or any other OPEC member state as well sporadic violence in Iraq could still cause short-term price spikes as we head into autumn when economic activity normally picks up," said Andrey Kryuchenkov of VTB Capital.