US President Barack Obama has said it would be unacceptable for Iran to have a nuclear weapon, but warned against a pre-emptive strike on the country.
He told The Atlantic magazine he was not bluffing by threatening US action if Iran built a weapon, but that an early strike could win Iran sympathy.
Iran says its nuclear programme is purely peaceful, but Israel and the US suspect it, of developing weapons.
There has been speculation that Israel might launch a pre-emptive strike.
Correspondents say, Washington is deeply alarmed by reports that Israel might take unilateral military action as early as April.
In the interview, Obama said he believed the Israeli government "recognizes that, as president of the United States, I don't bluff".