Greek government sources have said the country will default on a crucial repayment due to the International Monetary Fund later on Tuesday, plunging it deeper into financial crisis.
No resolution to the debt crisis is likely before a referendum on Greece's bailout on Sunday.
Investors kept a close eye on talks in Vienna on Iran's nuclear program that could end years of economic sanctions on the Islamic Republic and eventually allow a big increase in Iranian oil exports.
Iran and six world powers gave themselves an extra week to reach a final nuclear accord after it became clear they would miss a deadline on Tuesday, with U.S. and Iranian officials sounding upbeat even though obstacles remain.
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Diplomats and analysts have said a deal with Iran in Vienna is likely to be agreed, although it is not clear how long it would take to ease sanctions or how great an impact that would have on Iranian oil production or exports.
Asian imports of Iranian crude rose to the highest level this year in May at 1.2 million barrels per day, according to final figures on Tuesday. But they held at slightly above 1 million bpd for the first five months of the year, the level allowed under sanctions on Tehran.
The market awaited a U.S. government payrolls report on Thursday for clues on when the U.S. Federal Reserve could raise interest rates, the first such increase in about 10 years.