Brent crude fell for the fourth straight session, down 1.1% to below $112 a barrel.
The decline comes as finance ministers in the euro area delayed reaching an agreement on a bailout of the Greek economy.
Other commodities, with investors concerned about the knock-on impact for the global economy and demand, were also down.
Copper prices lost 1.3 percent, falling below $9,000 an ounce. Gold, popular in times of uncertainty, saw its prices hold steady at above $1,530 an ounce.
Euro zone finance ministers at the weekend postponed a final decision on extending a further €12bn in emergency loans to Greece, ratcheting up pressure on Athens to first impose harsh austerity measures.
They said they expected the money, the next tranche in last year's €110bn bailout package extended by the European Union and the International Monetary Fund, to be paid by mid-July.
'At the moment then, we are still in limbo - the EU and ECB appear to have reached a compromise on private sector involvement in the Greek bailout, but are still demanding austerity measures which are unacceptable to some in Greece,' said Kit Juckes, currency strategist at Societe Generale.
The euro was down 0.7 percent at $1.4205, edging back in the direction of a three-week low of $1.4073 hit last Thursday. The pound was worth €1.136 today.
The yield on 10-year Greek government bonds - the measure of concern about it debts - jumped 16 basis points to 17.69%, not far from a euro lifetime high of 18.90% hit on Friday.
