ATHENS—Greece's fiscal crisis took a new turn to violence Wednesday when three people died in a firebomb attack amid a paralyzing national strike, while governments from Spain to the U.S. took steps to prevent the widening financial damage from hitting their own economies.

U.S. Treasury officials have been quietly urging their European and International Monetary Fund counterparts to put together a Greek rescue plan more quickly to contain the damage, it emerged Wednesday, as U.S. policy makers worry the continent's problems...