Even as the United States and its European allies grapple with Russia¡¯s invasion of Ukraine and rising tensions with China, the smoldering crisis over Iran¡¯s nuclear program threatens to reignite.

In a sign of European concern, Britain, France and Germany have warned Iran they would trigger a return of U.N. sanctions against Tehran if it enriched uranium to the optimal level for a nuclear weapon, three European officials said.

The threat, made last year in a previously unreported letter sent by the countries¡¯ foreign ministers, underscores Western fears that Iran could produce bomb-grade uranium of 90% purity.