Republicans vying to oust Barack Obama from the White House ridiculed the Democratic president on Thursday for asking arch-foe Iran to "pretty please" return a downed US drone.

Obama acknowledged that Iran was holding the bat-winged RQ-170 Sentinel -- a stealth reconnaissance drone -- by saying Monday: "We've asked for it back. We'll see how the Iranians respond."

Former Massachusetts governor Mitt Romney, who has been at or near the top of the Republican pack fighting for the party's nomination to take on Obama in November elections, mocked Obama's request.

"This is a president that the spy drone being brought down, he says pretty please? Foreign policy based on pretty please? You have to be kidding," Romney said during a Fox News debate.

"This is a president who fundamentally believes that the next century is the post-American century. Perhaps it will be the Chinese century. He is wrong. It has to be the American century. America has to lead the free world."

Iran has also scoffed at the request.

Texas Governor Rick Perry said the United States should have destroyed or tried to obtain the craft in order to avoid Iran from obtaining sensitive US technology. He called Obama's reaction "the worst and the weakest."

The drone, designed to evade radar for surveillance flights, was on a CIA mission when it went missing on December 4, according to US officials. Iran has vowed to reverse engineer the drone but has given contradictory accounts of how the aircraft went down.

Libertarian-minded candidate Ron Paul, a frequent war critic, distanced himself from the other Republican candidates over how to handle growing tensions between Washington and Tehran.

Paul argued with fellow lawmaker Michele Bachmann over whether the United States can force Iran to stop developing a nuclear weapon, as the West and Israel fear the Islamic republic is doing under the guise of a civilian energy program.

Bachmann said she had "never heard a more dangerous" suggestion than Paul's proposal to work with Tehran to halt its nuclear ambitions, warning the move would risk American lives and "wipe our ally Israel off the face of the map."

She said Iran was led by "avowed madman" Mahmoud Ahmadinejad bent on the destruction of Israel.

An animated Paul dismissed Bachmann's "absurd" and "dangerous talk."

"You cannot solve these problems with war," Paul said. "Only when we declare the war, win them and get them over with."

The exchange came as Washington pushes for stricter sanctions to punish Tehran over its nuclear activities and amid speculation Israel may be weighing possible preemptive military action.