Obama accused Republican "interfering" on nuclear talks.

He said the 47 senators made an "unusual coalition" with Iran's hard-line religious leaders.

The letter reminds Iran that any deal is just an executive agreement unless it gets congressional approval.

Talks on Iran's nuclear programme are at a critical stage, with an outline agreement due on 31 March.

 

Last week Israel's Prime Minister Benjamin Netanyahu told Congress the deal currently being negotiated could "pave Iran's path to the bomb".

Separately, officials confirmed that US Secretary of State John Kerry would meet his Iranian counterpart, Mohammad Javad Zarif, on Sunday in Switzerland, as part of the process.

The P5+1 group of major powers - the US, UK, France, Russia and China plus Germany - is seeking to persuade Iran to curb its nuclear programme in exchange for an easing of UN sanctions.

They are trying to address concerns that Iran is seeking nuclear weapons technology, something Tehran denies.

Republicans and some Democrats have long been pushing for Congress to get a vote on any deal.

But the White House insists such an agreement does not require the approval of legislators, the BBC's Gary O'Donoghue reports from Washington.

Senator Tom Cotton, an Arkansas Republican, was the lead author of the letter to Iranian leaders

 

"I think it's somewhat ironic to see some members of Congress wanting to make common cause with the hardliners in Iran," Mr Obama said in response to the letter. "It's an unusual coalition."

He added that he would concentrate his efforts on trying to strike a deal.

Earlier, White House Press Secretary Josh Earnest said the letter interfered with diplomatic negotiations. He called it a "rush to war, or at least the rush to the military option".

Mr Zarif dismissed the letter as a propaganda ploy, adding that if a future administration revoked a deal it would amount to a blatant violation of international law.

In their letter to Iran, published on the website of Senator Tom Cotton, the senators suggest Iran's leaders "may not fully understand our constitutional system".

Barack Obama described the Republicans' open letter to Iran's leaders as "somewhat ironic"

They note that any agreement without their support would exist solely between President Obama and Iranian Supreme Leader Ayatollah Khamenei.

"The next president could revoke such an executive agreement with the stroke of a pen, and future Congresses could modify the terms of the agreement at any time," the senators wrote in the letter.

The signatories add that "most of us will remain in office well beyond [January 2017]" when Mr Obama's second term comes to an end.