SOEST, Germany — German Chancellor Angela Merkel said Monday she won’t release Greek rescue funds until the country shows it has a “sustainable, credible” plan to cut its budget deficit. A decision may be in a “few days,” she said.

Greek bonds plunged Monday, sending yields to the highest since 1998, amid concern Germany is delaying approval of a $60 billion rescue plan that would be co-financed by the euro region and the International Monetary Fund. The extra yield that investors demand to hold Greek 10-year debt over German bunds climbed 75 basis points to 634 basis points.

“We’re not doing this because we believe Greece needs help,” Merkel said at an election rally Monday in the western German town of Soest. “We’re doing it because we’re interested in the euro’s stability. We can’t idly stand by when our currency comes under threat.”

Merkel, in earlier remarks to reporters in Berlin, said there would be no decision on aid for Greece until the IMF works out a three-year deficit-reduction plan with the Greek government. She said Germany would assist Greece only after the government in Athens agrees to take “tough” measures.

Opposition lawmakers accused Merkel of playing politics, holding up an unpopular decision to shore up support in Germany’s most populous state, North Rhine-Westphalia which holds elections May 9. The vote will determine whether Merkel’s coalition will keep a majority in the federal parliament’s upper chamber.

The German government will implement a “fast-track” parliamentary approval process for Greek aid that may begin next week once the IMF has finished its review, Finance Minister Wolfgang Schaeuble said Monday.

In a statement Monday, Greek Finance Minister George Papaconstantinou confirmed the need to “over the next years reduce deficits drastically, control its debt and proceed with all the structural adjustments so that we will have a more competitive economy.”

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