BRUSSELS – The leaders of the 17 countries that use the euro scrambled Thursday night to consider emergency measures to lower the borrowing costs of Italy and Spain. The success of a summit meant to reassure markets hung in the balance.

An agreement by the 27 leaders of the European Union to spend $149 billion to stimulate economic growth was thrown into flux after Italy and Spain said they would block it unless it was paired with action to help lower the interest rates on their government bonds.

“We see it as a package,” said an Italian official on the condition of anonymity because the closed-door talks are ongoing.