US Defence Secretary Robert Gates said on Friday military action against Iran would only "buy time," adding diplomacy was needed to persuade the Islamic state to abandon its nuclear program. "The reality is there is no military option that does anything more than buys time. The estimates are one to three years or so," Gates said in an interview with a US channel. "The only way you end up not having a nuclear capable Iran is for the Iranian government to decide their security is diminished by having those weapons."