Image copyright AP Image caption Obama spoke to reporters about the letter during a meeting with European Council President Donald Tusk

US President Barack Obama has criticised a letter from Republican senators to Iran, accusing them of "interfering" in ongoing 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.

Image copyright Getty Images Image caption 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".

Media playback is unsupported on your device Media caption 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.

Nuclear Iran: What world powers want - and what they fear

Image copyright AFP

World powers imposed sanctions on Iran because they felt it was not being honest about its nuclear programme and was seeking the ability to build a nuclear bomb.

Tehran denied this. Talks between Iran and six world powers known as the P5+1 have tried to allay the suspicions in exchange for easing the sanctions

Specifically, the world powers want to curtail Iran's ability to enrich uranium, which can be used to make reactor fuel but also nuclear weapons

Disagreement centres on how to limit Iran's development and use of centrifuges that enrich uranium

Faster enrichment would cut the time Iran would need to produce enough weapons-grade uranium for a weapon, were it to choose to do so. The US wants this "break-out window" to be at least a year long

It is not known if Iran has a warhead or suitable delivery system

Iran's main nuclear sites

Democrats including Senate Minority Leader Harry Reid denounced the letter.

"Let's be clear," Mr Reid said on the floor of the Senate on Monday. "Republicans are undermining our commander in chief while empowering the ayatollahs."

The letter comes shortly after Congress invited Israeli Prime Minister Benjamin Netanyahu to speak on the issue of the Iran talks.

That move earned White House disapproval as Congress acted unilaterally.

Republicans now control both chambers of Congress after winning elections last November, giving them considerable leverage over Mr Obama.