FRANKFURT — The European Central Bank signaled on Sunday that it would intervene more aggressively in bond markets to protect Spain and Italy, and leading finance ministers conferred about the mounting threats to the world’s economies, as policy makers sought to calm markets unnerved by deteriorating public finances and slow economic growth.

The shock of the downgrade Friday of long-term United States government debt and the worsening situation in Europe added new urgency to the efforts to restore confidence and prevent an extension of the stock market slide that began last week.

After conducting an emergency conference call late Sunday, the European Central Bank said it would “actively implement” its bond-buying program to address “dysfunctional market segments,” while welcoming efforts by Spain and Italy to restructure their economies and cut spending. The bank did not identify which bonds it would buy, but its statement is likely to be interpreted as a sign that it will intervene to prevent borrowing costs for the two countries from growing unsustainable.

The move was a concession that Europe’s previous efforts to stanch its debt crisis have fallen short, and underscored the importance of propping up Spain and Italy. Those two countries are central pillars of the euro zone, unlike the countries on the periphery — Portugal, Greece and Ireland — that have already received bailouts, and their collapse would threaten the euro currency and intensify the turbulence in world markets.