The European Commission on Thursday said that there was no provision under EU treaties to exit the euro without also leaving the European Union.
Commission spokeswoman Karolina Kottova refused all comment on the prospects of Greece quitting the euro.
But when asked if under the bloc's rule-book it was impossible to abandon the single market without also leaving the 27-nation union, she said: "The treaty doesn't foresee an exit from the eurozone without exiting the EU."
The Lisbon treaty, which came into force in December 2009, for the first time introduced an EU exit clause, but did not provide for an exit from the euro.
"Greece's place is in the eurozone and the instruments are in place, the agreements were concluded for this to continue to be the reality," she added at a briefing.
A spokesman for Economic Affairs Commissioner Olli Rehn meanwhile said eight billion euros in loans for Greece this month would be held back until Athens meets its commitments.
The loan, part of a 110-billion-euro rescue of Greece from May last year, "cannot take place while there is no certainty on the Greek authorities' willingness to hold to pledges made just a few days ago."
He also said the International Monetary Fund (IMF) still needed to commit to its share in the loan.