Oil prices traded lower on worries over the Greek debt crisis. Greek Prime Minister Alexis Tsipras called for a referendum on Saturday morning. The Eurogroup has not agreed to extend the Greek bailout programme.

The referendum is scheduled to be on July 05. Greeks will be asked to vote if they want to accept the aid proposal by the country's creditors.

Greece's parliament approved a referendum on yearly Sunday.

The Greek government Sunday announced its decision to shut the country's banks and to impose capital controls to avert the collapse of its financial system. Banks will be closed until July 06.

Daily cash withdrawals has been limited to 60 euros and payments and transfers abroad has been banned.

Investors expect the results of talks on the Iranian nuclear program. It is likely that talks will continue as there are doubts that all issues can be resolved before June 30. An agreement could put more pressure on oil prices as Iran is likely to boost its crude exports.

WTI crude oil for August delivery decreased to $58.45 a barrel on the New York Mercantile Exchange.

Brent crude oil for August dropped to $61.80 a barrel on ICE Futures Europe.