U.S. stocks extended their losses in heavy trading on Monday after a collapse in Greek bailout talks intensified fears that the country could be the first to exit the eurozone.

Greece appeared to confirm it was heading for a default after a government official said the country would not pay a €1.6 billon loan installment due to the International Monetary Fund on Tuesday.

The European Central Bank froze funding to Greek banks, forcing Athens to shut banks for a week to keep them from collapsing.