It says it prefers much tougher international sanctions, but Defence Minister Ehud Barak reiterated after his talks with Gates Monday that the military option had not been taken off the table.

The speculation further intensified after three Israeli Dolphin-class submarines sailed through the Suez canal last month, in what Israeli defence officials described as a naval drill aimed at showing Israel's strategic reach in the face of Iran.

US officials have sent out mixed signals as to whether it unequivocally opposes an Israeli strike in Iran.

Vice President Joe Biden told US broadcaster ABC earlier this month that Washington could not dictate to a sovereign nation what it could or could not do to safeguard its security. Obama himself then flatly denied this meant a 'green light' for Israel to attack Iran.

Israeli experts on Iran generally agree that a military strike would be difficult to carry out and would set Iran's nuclear programme back only a few years.