Standard & Poor’s threatened Monday to downgrade the credit rating of 15 euro-zone countries, piling pressure on the currency union’s leaders to take radical steps to resolve their debt crisis at a summit later this week.

The decision to put 15 euro- zone countries — including AAA-rated nations such as Germany and Luxembourg — on watch for a possible cut in their credit worthiness also threatens to throw the euro zone’s bailout mechanism into disarray, since the rescue fund relies on those countries’ stellar rating to cheaply raise money on the markets.

“Today’s CreditWatch placements are prompted by our belief that systemic stresses in the eurozone have risen in recent weeks to the extent that they now put downward pressure on the credit standing of the eurozone as a whole,” S&P said in a statement shortly after markets closed in the United States.

The only two euro nations not put on credit watch were Cyprus, which was already under review, and Greece, which already holds the world’s worst rating.

The announcement came just hours after French President Nicolas Sarkozy and German Chancellor Angela Merkel revealed sweeping plans to change the European Union treaties in an effort to keep tighter checks on overspending nations. The proposal is set to form the basis of discussions at a summit of E.U. leaders on Thursday and Friday that is expected to provide a blueprint for an exit from the crisis.

While the Franco-German plan would tie the 17 euro-zone nations closer together, a tighter union would likely also result in heavier financial burdens for the region’s stronger economies, which have put up billions of euros to rescue Greece, Ireland and Portugal.

Analysts also noted that the proposals did not foresee a clear roadmap on how to get the euro- zone economies growing again and to reduce funding costs for struggling nations in the long term.

S&P said a rising risk of another recession in the currency union was one of the reasons behind its decision.

But the rating agency also appeared skeptical of the Merkel-Sarkozy plan and the ability of the euro zone as a whole to agree on new measures to fight the debt crisis, citing “continuing disagreements among European policy makers on how to tackle the immediate market confidence crisis and, longer term, how to ensure greater economic, financial, and fiscal convergence among eurozone members.”

France and Germany, the euro zone’s two largest economies which are both rated AAA, quickly came out against the S&P move.

“Germany and France reaffirm that the proposals they made jointly today will reinforce the governance of the euro area in order to foster stability, competitiveness and growth,” they said in a joint statement. “France and Germany, in full solidarity, confirm their determination to take all the necessary measures, in liaison with their partners and the European institutions to ensure the stability of the euro area.”

The euro fell after the S&P announcement, trading down 0.1 percent at $1.339, and trading in futures on the S&P 500-stock index and Dow Jones industrial average turned negative. Asian stocks dropped in early trading Tuesday. Japan’s Nikkei 225 index ended its morning session down 0.8 percent.

— Associated Press