A U.S. Coast Guard cutter rescued six Iranian mariners from a vessel in distress in the Persian Gulf, the second such incident in a week of tension punctuated by the Islamic republic’s death sentence to a young Iranian-American man and Secretary of State Hillary Rodham Clinton’s sharp criticism Tuesday of Tehran’s uranium enrichment activity.

The rescue was another reminder of U.S. efforts to demonstrate the humanitarian value of its naval presence in the Gulf, a strategic waterway that the Iranian government has threatened to close in retaliation for international sanctions over its nuclear program — a dispute which only shows signs of escalation.

On Tuesday, Clinton condemned Iran for enriching uranium at an underground bunker to a level that can be upgraded more quickly for use in a nuclear weapon than the nation’s main stockpile. She said Tehran was breaking its international obligations and demonstrating a “blatant disregard for its responsibilities.”. . .