US Secretary of State John Kerry has told supporters of Israel that Iran will not be allowed to obtain nuclear weaponry.
Speaking at the Aipac conference of Jewish leaders in Washington DC, Mr Kerry said the US would not tolerate a nuclear Iran but was determined to explore all diplomatic options.
"Those who say strike and hit need to check what might happen after we do that," he said, apparently warning against an Israeli pre-emptive strike on Iranian facilities.
“Let me sum up President Obama’s policy. Unequivocal. We will not permit Iran to obtain a nuclear weapon. Period.”
Mr Kerry said the US would not accept a "bad deal" with Tehran and would prefer to have "no deal" under such circumstances.
Turning his attention to the Israel-Palestinian peace talks he has been overseeing, he added: "I also believe we are at a point in history that requires the United States, as Israel’s closest friend and the world’s preeminent power, to do everything we can to help end this conflict once and for all."
The US would not allow the West Bank to "turn into another Gaza", he added.
He also expressed an unequivocal position on boycotts of Israel, saying it was essential to "stand together and with a single voice reject arbitrary and unwarranted boycotts.
"For more than 30 years, I have staunchly, loudly and unapologetically opposed boycotts of Israel... that will never change".
Speaking of the special relationship between the US and Israel, he said the two nations "like no other two countries on the planet...set out to be examples to the world".
A delegation of British Jews is attending the annual conference, including Jewish Leadership Council interim chief executive Simon Johnson and Jonathan Arkush, vice-president of the Board of Deputies.
Philip Rosenberg, the Board's public affairs director, is also in Washington, and following Mr Kerry's speech tweeted: "Good words John Kerry. No go make the deal we all crave."