The US defence secretary, Leon Panetta, has warned critics of Barack Obama's Iran policy not to mistake a willingness to pursue diplomacy for weakness.

Panetta, speaking to a largely sceptical audience at the annual conference of the powerful pro-Israel lobby in Washington, derided the aggressive posturing of some of the president's opponents and more hawkish supporters of Israel, who have pressed for an explicit commitment to the use of force against Iran by setting "red lines" that Tehran's nuclear programme must not cross.

Panetta said the military option is on the table as a last resort if sanctions fail, and the president's record demonstrates he will use it if he believes there is no alternative.

"As the president made clear, the United States does not bluff. In this town it's easy to talk tough. Acting tough is a hell of a lot more important," he told the the American Israel Public Affairs Committee (Aipac) conference.

"The president ordered 30,000 additional troops to battle in Afghanistan to confront a resurgent Taliban. He launched a comprehensive precision bombing campaign to protect the Libyans and ultimately toppled a brutal dictator. He has ordered US warships to pass through the straits of Hormuz – despite the threats that we have received from Iran.

"And he has been the driving force behind the most successful and lethal counter-terrorism campaign in US history, culminating in the bold decision to send US special operations forces hundreds of miles into Pakistan to take the risk to take down Bin Laden. And he did."

Panetta's speech was a clear rebuke to other speakers at Aipac, including three of the four Republican presidential candidates who also addressed the conference on Tuesday. Ron Paul was not invited following his calls to cut off aid to Israel, along with every other country, and because of his criticism that sanctions on Iran are driving it toward developing a nuclear weapon.

The other candidates also came out swinging against Obama's Iran strategy.

Mitt Romney called Obama's policy of "engagement" with Tehran naive, and said it gave the Iranian leadership time to develop its nuclear programme.

"Hope is not a foreign policy. The only thing respected by thugs and tyrants is our resolve backed by our power and our readiness to use it," he said.

"In recent days and weeks we've heard a lot of words from the administration. Its clear message has been to warn Israel to consider the costs of military action against Iran. I don't believe we should be issuing public warnings that create distance between the United States and Israel."

But in the end, Romney's position was not so far from Obama's.

"I will bring the current policy of procrastination toward Iran to an end. I will not delay in imposing further crippling sanctions," he said. "As president I'll be ready to engage in diplomacy but I will be just as ready to engage our military might."

Newt Gingrich, also speaking via video link, went furthest in saying that as president he would give Israel the means to attack Tehran's nuclear facilities and let it do so without question.

"I will initiate a strategy in the tradition of Reagan, Thatcher and Pope John Paul II to undermine and replace the Iranian dictatorship by every possible method short of war in order to achieve a government we could trust and could deal with," he said.

"At the same time I would provide all available intelligence to the Israeli government, ensure that they had the equipment necessary and reassure them that if an Israeli prime minister decides he has to avoid the threat of a second Holocaust through preemptive measures that I would require no advance notice to understand why I would support the right of Israel to survive in a dangerous world."

Rick Santorum was the only candidate to appear in person at Aipac.

"As I've sat and watched this play out on the world stage I've seen a president who has been reticent. He says he has Israel's back. From everything I've seen from the conduct of this administration, he's turned his back on the people of Israel," Santorum said to applause.

He accused Obama of appeasement over today's news that the US will join Britain, France, China, Russia and Germany in a fresh round of negotiations with Iran.

"Another appeasement, another delay. Another opportunity for them [Iran] to go forward while we talk," he said.

Santorum – breaking with the tradition that the president's opponents do not generally side with other powers on the question of foreign policy – said there was a "tragic disconnect" between how Obama and the Israeli prime minister, Binyamin Netanyahu, viewed the urgency of the situation.

"We need to set forth a clear ultimatum to the Iranian government. We need to say to the Iranian government: the time is now – you will stop your nuclear production now, you will open up your facilities for inspectors from the United States ad other countries so we can certify that those efforts are stopping and being dismantled, now," he said to strong applause from the Aipac audience.

"We need to put that ultimatum in place and we need to be prepared if that ultimatum is not met … that if they don't tear down those facilities, we will tear down them ourselves."

The Republican candidate also had stinging criticism for the US military chief, General Martin Dempsey, who called Iran a "rational actor".

"Rational actors don't call for the destruction of other states, call them cancers, preach radical theologies," he said. "Rational actors do not develop nuclear capability, calling for nuclear power, when they have hundreds of years of oil and gas to provide for their power and their medical research."

Romney broke with the focus on Iran to mention the Palestinians – who have been virtually invisible as an issue at this Aipac conference to the gratification of the Israeli government – although his comments will have brought them little comfort.

"The current administration has distanced itself from Israel and visibly warmed to the Palestinian cause. It's emboldened the Palestinians. They're convinced that they can do better with America directly than they can at the bargaining table with Israel," he said.

Romney took a stab at Obama's assertion last year that a two-state solution will be based on the 1967 armistice lines with land swaps – a statement long accepted as the basis of a deal but which brought a torrent of accusations from the Republican right and some of Israel's more militant supporters that he was selling out the Jewish state.

"I've seen Israel by land and by air. I've seen its narrow waist and its vulnerability," said Romney. "I would never call for a return to the indefensible '67 lines because I understand that, in Israel, geography is security."