Republican US presidential candidates have redoubled their public calls for "covert" operations against Iran and Syria, including sabotage, assassination and aid to opposition forces.

Former House speaker Newt Gingrich, who has led the calls for secret war, told a gathering of party activists on Wednesday he would use "covert capability" to bring about "regime replacement" in Tehran.

"They only have one very, very large refinery. I would be focused on how to covertly sabotage it every day," he told the Republican Jewish Coalition, a group highly critical of President Barack Obama's handling of ties to Israel.

Gingrich said US policy towards Syria must be to "replace" President Bashar al-Assad and "do everything we can, indirectly and covertly -- but without American forces -- to help" the opposition topple his government.

Former Massachusetts governor Mitt Romney, who recently lost to Gingrich the mantle of front-runner for the party's nomination to take on Obama in November 2012, called for Washington to secretly help dissidents in Iran.

"We should also have covert and overt activities to encourage voices of dissent within the country. Ultimately regime change is what's going to be necessary in that setting," he told the group.

One of their long-shot rivals, former senator Rick Santorum, told the same audience he hoped US assets were behind a recent deadly explosion at a missile base in Iran and vowed to put the world on notice of secret US operations.

"We need to say very clearly that we will be conducting covert activity to do everything we can to stop their nuclear program. And that means using covert activity like may have occurred at the missile site," he said.

"We need to be very clear: Any foreign scientists working in Iran on this nuclear program will be termed an enemy combatant and will be subject -- like any other enemy combatant, like Osama bin Laden -- to being taken out by the United States government as a threat to this country," he said.

In the same breath, Santorum pointed to the May raid in Pakistan in which US commandos killed bin Laden to accuse Obama of "not being able to keep a secret of anything good that he did for even more than 24 hours."

Gingrich proposed at a November 12 debate that Washington kill Iranian scientists and disrupt Tehran's suspect nuclear program -- "all of it covertly, all of it deniable."

In that same forum, Santorum said the United States must do "whatever it takes to make sure" Iran does not develop a nuclear program -- then wondered whether Washington may already be heavily involved in doing just that.

"There have been scientists turning up dead in Russia and in Iran. There have been computer viruses. There have been problems at their facility. I hope that the United States has been involved with that," he said.

"I hope that we have been doing everything we can, covertly, to make sure that that program doesn't proceed," he said.

Texas Governor Rick Perry suggested at a November 22 debate that Washington had many ways to put pressure on Assad's regime -- "overt, covert, economic sanctions."

"This is the time for us to use not only sanctions, but covert actions within Syria, to get regime change there," said Romney.

"There are people in the military that are shifting over, that are becoming part of the rebel effort. We should support those efforts," he added.

The pronouncements of the Republican presidential hopefuls have raised eyebrows among some career national security officials.

"The chances of success go down dramatically when you tell the world that is the major tool in your foreign policy bag of tricks," one former senior official in Republican president George W. Bush's White House told AFP.

The official said Bush's team "took a lot of heat for keeping secrets."

The official praised Obama's "very strong national security team," and urged Republicans not to view covert operations as "some kind of magic elixir that will cure all of the problems."

"Yes, the Obama victory dance after the Bin Laden raid cost us the opportunity to take full advantage of the information gathered; but do you think it would have been different with a GOP administration? Please," said the official, who requested anonymity in order to speak candidly.