Washington, March 10 (IANS/EFE): Almost 50 Republican senators warned Iran in a letter made public on Monday that an agreement on its nuclear programme that is not approved by the US Congress could lapse as soon as President Barack Obama leaves the White House in less than two years.

The letter containing the rather unusual warning was signed by 47 Republican senators -- including potential presidential hopefuls Marco Rubio, Ted Cruz and Rand Paul -- and was directed at the "leaders" of the Iranian regime.

"We will consider any agreement regarding your nuclear-weapons programme that is not approved by the Congress as nothing more than an executive agreement between President Obama and Ayatollah Khamenei," said the senators in the missive.

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

The senators noted that, according to the US constitution, the president has the authority to negotiate international agreements, but it is Congress that plays the key role in ratifying them.

Iran and representatives of the 5 1 Group -- that is, the US, China, France, Britain and Russia, plus Germany -- last week finalised the latest round of negotiations to achieve a pact on Iranian nuclear development.

Both parties acknowledged that there had been progress at the meetings, which are continuing on the premise that Iran will reduce its nuclear capability to avoid international suspicions that it might acquire nuclear weapons in exchange for the lifting of economic sanctions imposed on Tehran by the West.

The deadline for achieving a pact at the talks is at the end of June, but both sides have said that, in order to meet that timetable, they would need to reach a draft agreement by the end of March.

In an interview with CBS broadcast on Sunday, Obama said it was possible that a pact with Iran could be achieved if it can be "verified" that Tehran is not developing nuclear weapons.