MADRID—Spain made its most explicit suggestion yet that it would seek help from Europe for its struggling banks, as the country's budget minister said high interest rates on Spanish bonds were a signal the government risks losing access to financial markets.

Euro-zone officials worked privately to develop an approach to deal with Spain's intensifying financial troubles ahead of a European Union summit in Brussels at the end of the month, according to people familiar with the effort. The talks came as European officials have stepped up efforts to develop a road map for a possible tighter economic union within the common currency area, a goal that is still largely theoretical and distant.

Some leaders of the Group of Seven leading industrialized nations, meanwhile, pressed European leaders to act more aggressively to tame their debt troubles and contain Spain's problems.

The crisis in Spain, the euro zone's fourth-largest economy, is seen in financial markets as the acid test for the survival of the European common currency. Spain's troubles—unlike those of Greece, whose economy is one-fifth the size—can't be blamed on reckless government spending, but instead on a bursting real-estate bubble. The country is also widely considered too big to bail out, presenting Germany, which holds the euro-zone's purse strings, with a gigantic headache.

Spain's borrowing costs have been above 6% for more than three weeks, levels widely considered unsustainable over time. Investors have retreated from the bond market amid fears that Madrid will have to make good on some of the huge losses that the country's banks are expected to suffer on their property-loan portfolios—at a time when its budget is already under pressure from a shrinking economy and rising unemployment.