Action against Iran no bluff, says Obama

US President Barack Obama has delivered his starkest threat so far over Iran's nuclear ambitions, saying the US was "not bluffing" when it warned military action could be used to stop Tehran acquiring the bomb.

Raising the spectre of a "nuclear arms race in the most volatile region in the world", Obama predicted that if Tehran's apparent ambitions succeeded, four or five countries in the Middle East would scramble to acquire nuclear weapons.

Speaking before critical talks in Washington with Israeli Prime Minister Benjamin Netanyahu, Obama moved to delay Israel from attacking Iran by stressing the US was prepared to "watch Israel's back". "Both the Iranian and Israeli governments recognise that when the US says it is unacceptable for Iran to have a nuclear weapon, we mean what we say."

He dismissed Israeli fears that the US might resign itself to containing a nuclear Iran, telling The Atlantic magazine this ran "completely contrary" to his policy of nuclear non-proliferation. "I think that the Israeli government recognises that, as president of the US, I don't bluff," he said. "I also don't, as a matter of sound policy, go around advertising exactly what our intentions are." He emphasised the US's policy included diplomatic pressure, political isolation and "unprecedented and crippling" economic sanctions. "When I say we are not taking any option off the table we mean it. We will continue to apply pressure until Iran takes a different course."

He claimed the risk of an Iranian bomb being passed to terrorist groups was "profound", and that surrounding states would feel forced to follow suit.

"It is almost certain that other players in the region would feel it necessary to get their own nuclear weapons," he said. "So now you have the prospect of a nuclear arms race in the most volatile region in the world."

Obama is due to meet Netanyahu tomorrow amid mounting tensions over Iran. While he is anxious to avoid being sucked into a fresh conflict months before the US presidential election, Israeli officials have threatened to pre-emptively strike Iran's nuclear facilities unless Washington delivers a more bellicose warning to Tehran.