BRUSSELS (AP) -- The borrowing costs of both Spain and Italy dropped sharply in early trading Monday after the European Central Bank signaled it would intervene in the markets to keep the two countries' bond prices supported.

The yield on Italy's 10-year bonds was down 0.44 percentage points to 5.56 percent while the equivalent rate on Spain's tumbled 0.66 percentage point to 5.38 percent.

Growing concerns about the two countries' high government debt levels and lackluster economic growth pushed their borrowing costs to above 6 percent last week -- rates that are deemed unsustainable in the long-term for the eurozone's third and fourth largest economies.

Italy's main stock index, Milan's FTSE MIB also jumped at the open, after recording deep losses last week.

The positive market movements are a sign that investors were somewhat relieved by pledges from the ECB and policymakers around the world that they would do everything to stabilize financial markets. A Standard & Poor's downgrade late Friday of U.S. long-term debt added to the uncertainty over the weekend.

Most crucially for Europe, the ECB late Sunday decided to “actively implement” its bond-buying program, one of its main crisis tools that it had so far not used for Italy and Spain. The ECB has bought just under (euro) 80 billion ($113 billion) in Greek, Irish and Portuguese bonds so far, but had been reluctant to get involved in the much larger Italian and Spanish markets.

Buying bonds supports their prices, taking pressure off the issuing countries during an investor sell-off.

Bailing out Spain and Italy -- as it has done with Greece, Ireland and Portugal -- would likely overwhelm the 17-country eurozone.