US President Barack Obama responded to a letter sent by Republican senators to the Iranian leadership, saying calling it "somewhat ironic", saying "it's an unusual coalition" to see US Republicans talking directly to Iranian hard-liners.
Meanwhile, Iran's Foreign Minister Mohammad Javad Zarif on Monday dismissed as of "no legal value" a letter from 47 US senators warning that any nuclear deal will require Congressional approval.
In an open letter to the Islamic republic, the 47 Republicans, including Senate leaders and several potential 2016 presidential candidates, reminded Iranian leaders that President Barack Obama is in office only until January 2017, and a successor could scrap the agreement if Congress has not approved it.
"We believe that the letter has no legal value and is propaganda," Zarif said, quoted in Iranian media.
"The senators must know that under international law, Congress cannot change the content of the agreement.
"Any congressional action to prevent the implementation of any agreement will violate the international commitments of the (US) government.
"The world is not just in America," Zarif added.
The letter appeared to be another bid to influence or even derail the talks underway between Tehran and the P5+1 group of world powers to rein in Iran's contested nuclear program.
It marked a rare foray by Congress into US foreign policymaking, as negotiating with foreign governments is a responsibility typically handled by the executive branch, not lawmakers.
In an open letter to Iranian leaders, freshman Sen. Tom Cotton and 46 other Republicans said that without congressional approval any deal between Iran and the US would be merely an agreement between President Barack Obama and Iran's supreme leader, Ayatollah Ali Khamenei.
"The next president could revoke such an executive agreement with the stroke of a pen," they wrote, "and future Congresses could modify the terms of the agreement at any time."
The US and other nations are seeking a pact that would let Western powers verify that Iran will not obtain a nuclear weapon.
At the White House, spokesman Josh Earnest said the Republican letter interferes with negotiations over limiting Tehran's nuclear ambitions.
"I would describe this letter as the continuation of a partisan strategy to undermine the president's ability to conduct foreign policy and advance our national security interests around the globe," Earnest said. "The rush to war or at least the rush to the military option that many Republicans are advocating is not at all in the best interest of the United States."
Earnest said the talks with Iran are no different from the negotiations that resulted in an agreement with Syria to eliminate its chemical weapons arsenal. Earnest noted that Congress did not have to approve that agreement.
Though the Republican letter was addressed to leaders in Tehran, it seemed as much aimed at delivering a message in the United States.
Republicans and some Democrats want Congress to vote on any agreement. The pact the bargainers are working on would not require congressional approval because it is not a treaty. A treaty would require a two-thirds majority Senate vote to be ratified.
Dick Durbin, the No. 2 Democrat in the Senate, accused the Republicans of risking another war in the Middle East.
"This is a cynical effort by Republican senators to undermine sensitive international negotiations - it weakens America's hand and highlights our political divisions to the rest of the world," Durbin said in statement. "Understand that if these negotiations fail, a military response to Iran developing their nuclear capability becomes more likely. These Republican senators should think twice about whether their political stunt is worth the threat of another war in the Middle East."
Last week, Republican Senate Majority Leader Mitch McConnell, postponed action on legislation to give Congress a vote on any deal that emerges. He delayed the measure in the face of solid Democratic opposition to moving ahead on the bill now, just weeks before an end-of-March deadline for negotiators to produce an outline of an agreement.
The letter released Monday was signed by 47 of the Senate's 54 Republicans. Included were McConnell and the rest of the Senate leadership plus presidential contenders Marco Rubio, Rand Paul and Ted Cruz.