US Vice President Joe Biden issued a strong reprimand against a group of Republican senators late on Monday over their controversial decision to intervene in sensitive nuclear negotiations currently being hammered out between Tehran, Washington, plus five other world powers.

"The decision to undercut our president and circumvent our constitutional system offends me as a matter of principle," Biden said in a statement.

Unlike a reproval from President Barack Obama, Biden's words as vice president hold a different weight in Congress' upper chamber. The US vice president exercises authority in both the executive and legislative branches of government. In Congress, the vice president presides over the Senate and, additionally, holds the power to cast a tie-breaking vote in the 100-seat assembly.

Biden himself served nearly four decades as a senator, a point which he emphasized in his reproval of the Republican band.

"I cannot recall another instance in which senators wrote directly to advise another country - much less a long-time foreign adversary - that the president does not have the constitutional authority to reach a meaningful understanding with them," he wrote.

In a letter dated March 9, nearly 50 Republican senators, among them Majority Leader Mitch McConnell and several lawmakers eyeing the next presidential race, wrote to the Iranian government. In their open message, they warned Tehran that any nuclear deal signed with Obama would be a "mere executive agreement" if not approved by Congress, meaning that the next president could revoke it.

Dangerous diplomacy?

Talks aimed at curbing Tehran's nuclear capabilities are due to resume next week in Lausanne, Switzerland. There, US Secretary of State John Kerry is to meet his Iranian counterpart, Javad Zarif, as well as with representatives from the United Kingdom, France, Germany, Russia and China, also known as the "P5+1."

Iranian Foreign Minister Zarif rejected the Republicans' "propaganda ploy," as well as their false interpretation of a possible deal, pointing to the fact that many such deals were "mere executive agreements."

Although the initial Iranian reaction appeared dismissive toward the lawmakers' attempt to intervene in talks, the US vice president made a point of berating the group for its lack of political foresight.

"This letter sends a highly misleading signal to friend and foe alike that that our commander-in-chief cannot deliver on America's commitments - a message that is as false as it is dangerous," he said.

First Netanyahu, now the ayatollah

The decision by the Republicans to stray from the US' diplomatic framework was not the first time this year. Israeli President Benjamin Netanyahu addressed Congress in a speech last week upon an invitation by Republican House Speaker John Boehner.

The Israeli leader spoke to a fear held by some in Congress that the Obama administration was backing an accord that would only foster the growth of Iran's nuclear infrastructure, endangering the region. However, President Obama dismissed the speech as "theater."

The trend of undercutting the president on an international level has concerned fellow Congressional lawmakers, including Senate Democratic leader Harry Reid.

"Let's be very clear: Republicans are undermining our commander-in-chief while empowering the ayatollahs," Reid said on Monday.

President Obama, for his part, mocked his opponents' strategy of appealing to Iranians also opposed to negotiations: "I think it is somewhat ironic to see some members of Congress wanting to make common cause with hardliners in Iran."

"It is an unusual coalition," Obama said.

kms/gsw (AP, AFP, Reuters)