Republican White House hopefuls have slammed President Barack Obama's handling of Iran and vowed a tougher line -- even going to war -- to stop Tehran obtaining nuclear weapons.

"If we re-elect Barack Obama, Iran will have a nuclear weapon," former Massachusetts governor Mitt Romney said Saturday, as the candidates opened their first foreign policy debate. "If you elect me as the next president, they will not."

But former pizza chain executive Herman Cain, running neck-and-neck with Romney in the polls, said "the only way you can stop them is through economic means," pledging tougher sanctions and support for domestic opposition groups.

Former House speaker Newt Gingrich sided with Romney, calling for "maximum covert operations" against Iran's suspect nuclear program, "including taking out their scientists" and sabotage.

The International Atomic Energy Agency, the UN atomic watchdog, said Tuesday it had "serious concerns" based on "credible" information that Iran has "carried out activities relevant to the development of a nuclear explosive device."

But former Republican senator Rick Santorum said the United States "should be working with Israel right now to do what they did in Syria, what they did in Iraq, which is take out that nuclear capability before the next explosion we hear in Iran is a nuclear one -- and then the world changes."

Republican Representative Ron Paul said it was "not worthwhile" to go to war to halt Iran's suspect nuclear program, and compared the heated US rhetoric against Tehran to the flawed case for the March 2003 invasion of Iraq.

"I'm afraid what's going on right now is similar to the war propaganda that went on against Iraq. You know, they didn't have weapons of mass destruction, and it was orchestrated," he warned.

Gingrich and Romney criticized Obama on Syria, saying he had done too little to help protesters there, with President Bashar al-Assad's bloody crackdown on demonstrators claiming an estimated 3,500 lives so far.

They both urged covert action in Syria.

Most of the candidates sharply criticized Pakistan, with Texas Governor Rick Perry saying he would restrict foreign aid -- which is allocated by the US Congress -- to countries that do not support American interests.

Cain said Washington's ties to Islamabad "must be reevaluated" and said "we don't know, it's not clear" whether Pakistan is friend or foe of the United States.

Cain, Republican Representative Michele Bachmann, Santorum and Perry all said they would overturn Obama's ban on harsh interrogations of suspected terrorists widely seen as torture.

"If I was president I would be willing to use waterboarding," said Bachmann, referring to a method of simulated drowning.

Perry agreed. "I don't see it as torture," he said. "I see it as an enhanced interrogation technique," using the preferred term under Obama's predecessor George W. Bush.

However Paul, bluntly declaring that "waterboarding is torture," said: "It's illegal under international law and under our law. It's also immoral, and it's also very impractical."

Former US envoy to China Jon Huntsman sided with Paul, saying: "We diminish our standing in the world and the values that we project, which include liberty, democracy, human rights and open markets, when we torture."

The debate was the first since Gingrich surged in the polls, becoming one of the leading Republican candidates.

A McClatchy-Marist nationwide poll, conducted between November 8 and 10, showed that while Romney led among Republicans and Republican-leaning independents with 23 percent, Gingrich was now second at 19 percent.

Cain, who is battling allegations of sexual harassment, slipped to third place with 17 percent.

Of the eight Republican candidates, only Huntsman has extensive foreign policy experience.

The Republicans also tarred Obama as an unreliable friend to staunch US ally Israel -- hoping to energize their conservative Christian base and douse his support among Jewish voters, a key bloc in heavyweight states such as Florida.

Foreign policy can define a presidency but is unlikely to decide the election: US voters are focused foremost on the sour economy.

But Romney and Gingrich also took a hawkish line on Obama's decision to order the slaying of US-born key Al-Qaeda figure Anwar al-Awlaqi -- agreeing it was the right thing to do -- although Paul called into question its legality.

Asked about what are seen in Washington as Beijing's unfair trade practices, Perry vowed a hard line and predicted that China's communist government would "end up on the ash-heap of history" -- Republican icon Ronald Reagan's prediction about the Soviet Union.

"Like everybody else on the world stage, they have to play by the rules," said Romney, promising to seek retaliatory tariffs on Chinese goods over Beijing's alleged policy of keeping its currency -- and thus its products -- artificially cheap.

Obama presses China, Russia on Iran
Honolulu, Hawaii (AFP) Nov 12, 2011 - President Barack Obama Saturday canvassed the leaders of skeptical Russia and China after calling for new sanctions and isolation for Iran over revelations about its nuclear program.

Obama met Russian President Dmitry Medvedev and Chinese President Hu Jintao on a day of high-level diplomacy on the eve of the Asia-Pacific Economic Cooperation (APEC) summit in his home state of Hawaii.

Washington reacted to the release of an International Atomic Energy Agency (IAEA) report Tuesday citing "credible" evidence that Iran had worked towards nuclear weapons, with a call for more steps to isolate Tehran.

But Russia and China, which could block a stiffening of international sanctions as permanent members of the UN Security Council, responded coolly to the outrage expressed by the United States and its allies.

Obama said after talks with Medvedev that the two sides had "reaffirmed our intention to work to shape a common response so we can move Iran to follow its international obligations when it comes to its nuclear program."

But he steered clear of the sanctions debate in public. Medvedev told reporters that the two sides discussed Iran but offered no further details.

Obama also spoke publicly about Iran at a photo-op before his meeting with Hu, but he was even less specific, calling for "efforts to jointly ensure that countries like Iran are abiding by international rules and norms."

US officials said that there was no disagreement in the talks on the need for Iran to satisfy international norms and obligations on its nuclear program.

But they also said that Obama did not raise specifically the most divisive issue -- the call for more sanctions on Tehran.

"Both Russia and China said that they do not want to see the spread of nuclear weapons to Iran or frankly to any new state," said Ben Rhodes, a US deputy national security advisor.

"They remain committed to diplomatic efforts to tell Iran to live up to its obligations."

White House spokesman Jay Carney added: "there was no disagreement about the need for Iran to live up to its international obligations, or any discussion about disagreements about (the IAEA) report.

Officials said Friday that Washington also had the option of seeking new sanctions outside the Security Council with "like-minded states."

Tehran denies its program is meant to produce nuclear weapons. The IAEA report however said there was "credible" evidence to doubt its denials.

Russia's Deputy Foreign Minister Gennady Gatilov made clear last week that Moscow would resist the tougher sanctions no matter what the UN report said.

"Any additional sanctions against Iran will be interpreted by the international community as a means of changing the regime in Tehran," Gatilov told Interfax.

"This approach is unacceptable to us, and Russia does not intend to review this proposal," he said.

Russia has previously backed four rounds of UN Security Council restrictions on Iran while resisting the most crippling measures that could directly impact the two sides' military and energy ties.

Medvedev has also condemned Israel's warning that it was getting closer to launching a military strike on Iran for its suspected efforts to develop a nuclear bomb.

Winning Russia's cooperation for deepening the isolation of Iran was a centerpiece of the "reset" of relations engineered with Moscow by the Obama administration.

The policy also produced a new nuclear disarmament pact between the two old War foes and resulted in the United States reconfiguring its plans for missile defense in Europe that had angered Russia.

But some observers are now questioning whether the policy has peaked, given that Medvedev, with whom Obama established a personal rapport, is expected to step aside next year to allow hardline Prime Minister Vladimir Putin to return to the presidency.

China also appears to be reluctant to permit further UN sanctions on Iran, after foreign ministry spokesman Hong Lei said Thursday such measures "cannot fundamentally solve the Iran issue."

"Dialogue and negotiation are the right way out for the Iranian nuclear issue," he told reporters.