A senior US official on Tuesday said domestic politics in Iran were behind a charge by Tehran that Washington had flip-flopped on pursuing diplomacy to end a nuclear standoff.
The official, speaking on condition of anonymity, said that the White House was not surprised at the charge after President Barack Obama said that using force was still an option if diplomacy failed.
The complaint about a supposed US reversal was made by Iranian Foreign Minister Mohammad Javad Zarif on Twitter.
"President Obama needs consistency to promote mutual confidence," Zarif said.
"(A) flip-flop destroys trust and undermines US credibility."
The senior US official said that Washington understood that Iran had domestic political pressures that it must attend to as it embarks on a new round of nuclear talks with world powers in Geneva later this month on its nuclear program.
Washington's preferred option remained diplomacy to ease the nuclear showdown, the official said, and added that the administration believed Iran's new flexibility and offer to talk seriously about the nuclear program could be attributed to punishing sanctions that have ravaged its economy.
The exchange reflected the extent to which both Washington and Tehran face severe domestic constraints as they embark on a historic process of negotiation after years of deep mistrust.
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Iranian President Hassan Rouhani is seen as having only a limited amount of time to deliver on his election vows of easing sanctions and engaging the outside world, before hardliners in the cleric regime in Tehran begin to clip his power.
Obama faces deep skepticism among some powerful Democrats and Republicans in Congress over his engagement strategy, which could complicate any efforts to ease sanctions to induce concessions from Tehran, or as the condition of any eventual diplomatic deal to end the nuclear program.
The comments by Obama, which Zarif picked up, came after an Oval Office meeting with Israeli Prime Minister Benjamin Netanyahu on Monday.
"We take no options off the table, including military options," Obama said.
The president however said that after speaking to Rouhani last week by telephone, he believed a "resolution" was possible to the nuclear showdown, though large obstacles remained.
After meeting Obama on Monday, Netanyahu warned that sanctions must be maintained against Tehran and even strengthened if necessary, saying Iran had only agreed to negotiate because of the punitive measures.
But Zarif disagreed.
Obama's "presumption that Iran is negotiating because of his illegal threats and sanctions is disrespectful of a nation, macho and wrong," he said in another post on his Twitter account.