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 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, Mr Obama said he believed the Israeli government "recognizes that, as president of the United States, I don't bluff".

A female attendee at President Obama's campaign fundraiser in New York City on Thursday interrupted the events to shout about Iran.

"Use your leadership - no war on Iran!" she said,

Obama responded, "Nobody's announced a war, young lady, but we appreciate your sentiment. You're jumping the gun a little bit there".

Mr Obama re-iterated that "a military component" for dealing with Iran remained an option, as well as sanctions and diplomacy, but warned that a premature attack on Iran might be counterproductive.

"At a time when there is not a lot of sympathy for Iran and its only real ally [Syria] is on the ropes, do we want a distraction in which suddenly Iran can portray itself as a victim?"

Mr Obama is due to meet Israeli Prime Minister Benjamin Netanyahu in Washington on Monday.

The US president described his relationship with Mr Netanyahu - often depicted as frosty - as "very functional," despite the two leaders coming from different political traditions.

He said most of their differences were "tactical and not strategic".

Mr Obama was heckled on the issue of military action against Iran at a Democratic fundraising event in New York on Thursday evening.

American officials are thought to be seeking to persuade Israel to give sanctions more time to work.

The US has recently tightened its sanctions against Iran, imposing sanctions on the country's central bank and against three oil companies that trade with Iran.

The European Union has also adopted an oil embargo against Iran. The UN ratified four rounds of sanctions against Iran between 2006 and 2010 in reaction to its refusal to halt uranium enrichment and co-operate with the International Atomic Energy Association (IAEA).

In some of his toughest comments yet on Tehran's nuclear drive, Obama also warned that Israel and Iran should take seriously possible US action against Iranian nuclear facilities if sanctions fail to stop the country's atomic ambitions.

"I think that the Israeli government recognizes that, as president of the United States, I don't bluff," Obama told the Atlantic Monthly magazine in remarks published Friday.

"I also don't, as a matter of sound policy, go around advertising exactly what our intentions are. But I think both the Iranian and the Israeli governments recognize that when the United States says it is unacceptable for Iran to have a nuclear weapon, we mean what we say."

Netanyahu arrived in Canada early Friday ahead of key discussions Monday with Obama at the White House, against a backdrop of growing fears that Israel could unilaterally strike suspect Iranian nuclear facilities.

Tehran insists its nuclear program is for civilian purposes only but Western nations suspect the Islamic republic is leading a covert program to develop a nuclear weapons capability and is not far from achieving its goal.

Iran is the second-largest Organisation for Petroleum Exporting Countries (OPEC) producer, exporting up to 2.5 million barrels of oil a day. Around 450,000 of these barrels go to the European Union - the second-largest market for Iran after China. Tehran's threat to block the Strait of Hormuz, thus preventing one-sixth of the world's oil and 70% of OPEC's exports from reaching the market; no wonder oil traders are falling over themselves to lock up as much crude as they can