The leaders of the euro zone's three largest economies agreed on Thursday to pursue closer political and economic integration as the European Central Bank considered measures to shore up the region's banks and keep the protracted debt crisis from freezing new lending.

The push to renegotiate EU treaties adds yet another layer of complexity and tension to a crisis that has already stretched the euro zone's leadership to a breaking point. The crisis has toppled seven euro-zone governments in less than a year, including those in Spain, Italy and Portugal.

Proposals for treaty changes to subject countries to stricter oversight of their budgets and spending signal the bloc's leaders