US President Barack Obama said Washington was not ready to open an embassy in Iran due to differences over nuclear policy, a position that contrasts with his new approach to Cuba.

"I never say never, but I think these things have to go in steps," Obama said in an interview with National Public Radio that aired Monday, regarding re-opening the long-shuttered US mission in Tehran.

"Cuba is a circumstance in which for 50 years, we have done the same thing over and over again, and there hadn't been any change," Obama told NPR December 19 in the White House, shortly before he flew to Hawaii for a family vacation.

"And the question was, should we try something different with a relatively tiny country that doesn't pose any significant threat to us or our allies?"

By contrast, Obama said, Iran is "a large, sophisticated country that has a track record of state-sponsored terrorism, that we know was attempting to develop a nuclear weapon -- at least the component parts that would be required to develop a nuclear weapon."

Whether or not to improve US diplomatic ties and ease existing sanctions on Iran rests in large part on the outcome of current negotiations between global powers and Tehran on the Islamic republic's nuclear program, Obama added.

"If we can take that big first step, then my hope would be that that would serve as the basis for us trying to improve relations over time."

On November 24 Iran and the so-called P5+1 (The United States, Britain, China, Russia, France and Germany) agreed to renew their interim agreement from the previous year and extend their discussions until July 1, 2015 to obtain a final agreement that would prevent Iran's development of a nuclear bomb, in exchange for the lifting of international sanctions.

Obama announced on December 17 that Washington would reopen its embassy in Havana.