Merkel, who remained silent Sunday, will meet with party chiefs and parliamentary caucus leaders in Berlin on Monday afternoon, and then brief reporters, said Steffen Seibert, her chief spokesman. Vice Chancellor Sigmar Gabriel told broadcaster ZDF that the European Union is facing its biggest crisis since the establishment of its predecessor organization in the late 1950s under the Treaty of Rome.

Merkel spoke by phone with U.S. President Barack Obama on Sunday and the two leaders agreed it’s critically important to make every effort to return to a path that will allow Greece to stay in the euro, the White House said in a statement. The call was one of a flurry of discussions Sunday after Greek Prime Minister Alexis Tsipras over the weekend rejected the creditors’ conditions for aid and called a July 5 referendum on the proposals.

“Mr. Tsipras wants to accept all these offers only if Europe doesn’t link them to any conditions for reforms in Greece,” Gabriel, chairman of the Social Democratic Party, Merkel’s coalition ally, said in an interview with newspaper Sueddeutsche Zeitung. “Europe won’t be able to accept this even after a referendum.”

Gabriel canceled a scheduled trip to Israel set to begin Sunday to deal with the unfolding crisis, Deutsche Presse Agentur reported.

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