Iran's shunning of the Bahrain security conference accentuated the growing tension between the Islamic republic and the United States. But Iran's absence was also an indication of tension between Iran and its Gulf neighbor, the kingdom of Bahrain, a key U.S. ally. Bahrain is home the U.S. Fifth Fleet.

Addressing several hundred delegates attending a security summit organized by the London-based International Institute of Strategic Studies in Manama, the capital of the small Gulf kingdom of Bahrain, Gates said, "We must keep all our options open."

No official explanation was offered by Iran for its cancellation