Forty-seven Republican US senators have warned Iran's leaders that any nuclear deal with President Barack Obama could last only as long as he remains in office, an unusual partisan intervention in foreign policy that could undermine delicate international talks with Tehran.

The open letter was signed by all but seven of the Republicans in the Senate and none of Obama's fellow Democrats, who called it a "stunt". Iranian Foreign Minister Javad Zarif dismissed it as a "propaganda ploy" from pressure groups he called afraid of diplomatic agreement.

In the letter, the senators said Congress plays a role in ratifying international agreements. Noting Obama will leave office in January 2017, they said any deal not approved by Congress would be merely "an executive agreement" that could be revoked by Congress.

The White House said the letter was a partisan effort to undermine Obama's foreign policy by lawmakers who oppose a deal even if the only alternative is military action.

Obama said his focus now was on seeing if negotiators could get a deal or not, taking a jab at Senate Republicans for allying themselves with Iranian hardliners opposed to a deal.

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

A Western diplomat said the action was "without precedent". "It's 100 per cent an American issue, but obviously it could become a real problem," the diplomat said.

Iran's Zarif blasted the Republicans. "I wish to enlighten the authors that if the next administration revokes any agreement 'with the stroke of a pen' ... it will have simply committed a blatant violation of international law," he said in a statement.

DEMOCRATS NEEDED TO PASS LEGISLATION

The letter seemed to harden partisan lines in the Senate, where Republicans will need Democrats' support to pass legislation now in the works to tighten sanctions on Iran or require congressional approval of a deal.

"Republicans are undermining our commander in chief while empowering the ayatollahs," said Democratic Minority Leader Harry Reid.

The letter, first reported by Bloomberg, was the latest Republican effort to influence the Iran talks. Many Republicans worry Obama is so eager for a deal he will sign off on an agreement leaving Iran able to easily make a nuclear weapon.

Last week, Israeli Prime Minister Benjamin Netanyahu told Congress Obama was negotiating a "bad deal" after Republicans invited him to speak about Iran, without consulting the White House or Democrats.

World powers and Tehran are trying to reach a framework agreement this month, and a final deal by June, to curb Iran's nuclear program in exchange for easing sanctions. Iran denies its civil nuclear program is a cover for developing weapons.

Secretary of State John Kerry will travel to Switzerland on March 15 for the next round of talks.

The letter was spearheaded by first-term Senator Tom Cotton, who has called for "regime change" in Iran, not negotiations. Signers included all of the Senate's Republican leaders, and possible 2016 presidential contenders Ted Cruz, Marco Rubio and Rand Paul.

A spokeswoman for Cotton said his office had invited several Democrats to co-sign but none had done so.

One Senate Republican who did not sign was Senate Foreign Relations Committee Chairman Bob Corker. An aide said Corker is focused on getting a veto-proof majority to support his legislation, backed by both Republicans and Democrats, that would require Congress' authorisation of an Iran deal.