Stark warnings to Greece over its stalemated bailout talks have sent the Athens Stock Exchange sharply down, with the main index tumbling 3.8 percent minutes after opening Friday.

The International Monetary Fund took the toughest stance, saying Thursday its negotiators returned to Washington as there had been no progress in talks and major differences remained in key issues. Earlier, European Union President Donald Tusk had warned Greece there was "no more time for gambling."

Greek officials have sounded optimistic for a deal by the end of the month, when its 240 billion euro bailout expires and it loses access to the rescue loans it desperately needs to repay debts and avoid a default that could force it out of the euro. But its creditors have dampened those hopes.