The former House speaker said America should develop its own oil and gas resources, so as not to be dependent on oil from the Middle East, which would be compromised should Iran make good on threats to block the Straits of Hormuz.

Referring to the crackdown in Syria, where government forces have pounded the rebel city of Homs for nearly three weeks, killing dozens, he said he would ask America's allies to help oust the Syrian leader.

"We clearly should have our allies -- this is an old-fashioned word -- we have our allies covertly helping destroy the Assad regime. There are plenty of Arab-speaking groups that would be quite happy," he said.

"There are lots of weapons available in the Middle East," he added.

Three of the four remaining in the race spoke out in favor of making clear the military threat to Tehran should it continue to develop technology which Washington fears will result in the Islamic state having a nuclear weapon.

"Syria and Iran is an axis," said former Pennsylvania senator Rick Santorum, who is trying to bypass Mitt Romney to be the frontrunner in the Republican race to take on Democratic President Barack Obama in November elections.

"Syria is a puppet state of Iran. They are a threat not just to Israel, but they have been a complete destabilizing force within Lebanon, which is another problem for Israel and Hezbollah," said the Christian conservative.

Hitting out at Obama, he added: "This president obviously has a very big problem in standing up to the Iranians in any form... he's afraid to stand up to Iran."

Romney, struggling to fend off Santorum's poll surge, said: "I agree with both these gentlemen... You're seeing on the Republican platform a very strong commitment to say we're going to say no to Iran.

"It's unacceptable for them to have nuclear weapons. Syria is their key ally, the only ally in the Arab world, also their route to the sea. Syria provides a shadow over Lebanon."

He added: "We have very bad news that's come from the Middle East over the past several months, a lot of it in part because of the feckless leadership of our president."

"One piece of good news -- the key ally of Iran, Syria, has a leader that's in real trouble," Romney said, adding that the West should grab the opportunity to press for a change of leadership in Damascus.