NAUEN, Germany/ATHENS - The European Union's chief executive warned Greeks on Monday not to expect the euro zone to bow to leftist Prime Minister Alexis Tsipras' demands in a growing confrontation over Athens' debt crisis.

Tsipras laid out plans on Sunday to dismantle Greece's "cruel" austerity programme, ruled out any extension of its 240 billion euro international bailout, which runs out at the end of this month, and vowed to seek reparations from Germany for World War II.

His uncompromising maiden policy speech to parliament spooked European financial markets and partners outside the euro area. British Prime Minister David Cameron chaired a special meeting with finance ministry and Bank of England officials on Monday to plan for a possible Greek exit from the euro zone, a Treasury source said.

Ahead of a meeting of G20 finance ministers in Istanbul, the United States and Canada urged the EU and Greece to tone down the rhetoric over austerity and work for a compromise on the debt issue.

Greek Finance Minister Yanis Varoufakis, who found scant support on a tour of European capitals last week for his plans to restructure Greece's debt, angered some euro zone partners by saying on Sunday the 19-nation single currency area would ultimately collapse if Greece were forced out.

European Commission President Jean-Claude Juncker told reporters on a visit to Germany: "Greece should not assume that the overall mood has so changed that the euro zone will adopt Tsipras's government programme unconditionally."

The head of the EU's executive arm, who met the Greek leader in Brussels last week, said he did not expect a deal on the way forward with Greece at an EU summit on Thursday or a finance ministers' meeting of the euro zone on Wednesday.

"I don't think we'll reach final conclusions so soon," Juncker said at a meeting of Germany's Social Democratic Party (SPD) in Nauen, near Berlin.