US President Barack Obama said he was "embarrassed" for Republicans who signed a controversial letter warning Iran’s leaders about reaching a nuclear deal with the White House.

US President Barack Obama said he was "embarrassed" for Republicans who signed a controversial letter warning Iran's leaders about reaching a nuclear deal with the White House.

"I am embarrassed for them," Obama said in an interview with Vice media, which is expected to be released Monday.

"For them to address a letter to the Ayatollah who, they claim, is our mortal enemy and their basic argument to them is: don't deal with our president because you can't trust him to follow through on an agreement. It's close to unprecedented."

Forty-seven Senate Republicans -- including several potential 2016 presidential candidates -- signed the open letter to Iran's supreme leader published earlier this month.

Republicans warned any deal agreed before Obama leaves office in 2017 is "nothing more than an executive agreement between President Barack Obama and Ayatollah Khamenei."