BRUSSELS - Standard & Poor's is threatening to downgrade the credit rating of 15 eurozone countries, saying the worsening debt crisis is affecting the bloc's strongest economies.

The decision Monday to put 15 eurozone countries, including AAA-rating nations such as Germany and Luxembourg, on watch for a possible downgrade piles pressure on eurozone leaders to find a solution to the currency union's debt troubles at a summit later this week.

S&P said its decision was "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."

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.