New York's main contract, light sweet crude for delivery in October, was down $1.12 to $86.84 a barrel in morning trade and Brent North Sea crude for November slipped 81 cents to $111.41 a barrel.
Last week, eurozone finance ministers meeting in Poland, decided to delay until October a decision on eight billion euros ($11 billion) of bailout loans blocked until Greece persuades auditors it is on track to cut its deficit.
US Treasury Secretary Timothy Geithner, who also attended the meeting, and his German counterpart Wolfgang Schaeuble also disagreed over Europe's handling of the debt crisis.
The disagreements within Europe and with the United States could affect attempts to mount coordinated action to deal with the crisis before it gets out of hand and batters the global financial system.
Analysts said investors are also awaiting the results of a meeting on Tuesday and Wednesday of the US central bank's Federal Open Market Committee on interest rates.
Developments in the US economy are being closely watched by the energy market because the United States is the world's biggest oil consuming nation.
"The Fed is widely expected to discuss and announce further monetary policy measures," DBS Bank said in a market commentary.
It added however that "it is very unlikely that this discussion ends with no action being taken".