German Chancellor Angela Merkel said on Thursday that Greece has no choice but to carry out economic reforms if its wants to receive more financial aid, dashing any hopes Athens might have had for a softening in Berlin's stance.

Tensions over Greece's massive financial bailout overshadowed a European Union summit amid fears that the country could accidently drop out of the eurozone, triggering a crisis across the currency zone shared by 19 nations.

"There can only be a solution on the basis of the agreements reached in the eurogroup," Merkel said, referring to the meeting last month of finance ministers from the countries that use the euro.

Before Greece receives further rescue loans it must submit a detailed list of reforms, but the radical left-led leadership in Athens has been slow to come forward with new ideas.

A last-minute meeting of select leaders has been added to the European summit agenda for late Thursday to discuss the situation in Greece.

Merkel agreed, stressing that "it's absolutely clear that nobody can expect a solution for Greece's problems tonight" or on Monday, when she hosts Greek Prime Minister Alexis Tsipras in Berlin.

Greek Deputy Prime Minister Giannis Dragasakis said Tsipras would come to Brussels "with specific proposals".