Obama vows to veto new Iran sanctions

US President Barack Obama said he would veto proposed bipartisan legislation to impose new sanctions on Iran as long as diplomatic negotiations over a nuclear deal remain underway.

"I will veto a bill that comes to my desk," ABC News reported, citing Obama responding to a question at a joint press conference with visiting British Prime Minister David Cameron.

"There is no good argument for us to undercut, undermine the negotiations until they play out," Obama said.

Obama said hitting Iran's economy with new sanctions in the next 60 to 90 days would violate an interim agreement reached last year, deepen recriminations and heighten the risk of a military confrontation.

"Congress needs to show patience," Obama said.

If a deal is not reached, with assurances that Iran could not obtain a nuclear weapon, "I would be the first one to come to Congress to say we need to tighten the screws," he added.

Republicans and Democrats in the Congress are pushing for another immediate round of sanctions on the Iranian economy, a move led by some high profile White House allies.

At a Senate Democratic retreat Thursday in Baltimore, Obama reportedly clashed with Senator Robert Menendez of New Jersey who is leading the charge on new Iran sanctions, even as talks are ongoing.

While the gathering was closed to reporters, Menendez was said to have taken "personal offence" to Obama's suggestion that the legislation was politically motivated, sources said.

Obama Friday said the chances of a deal with Iran are "likely less than 50-50" - but argued that talks are the best chance in decades to resolve the nuclear standoff.

The next round of talks has already begun in the Swiss city of Geneva between Iran and US negotiators.

British Prime Minister Cameron also warned Congress against new sanctions and said he would personally call lawmakers to express the UK view.

"I have contacted a couple senators Friday, and I may speak to a few more this afternoon," he said.

"Simply to make the point that as a country that stands along side America in these vital negotiations that its our opinion that further sanctions won't help," the British PM added.

The major western powers and Iran have given themselves time until late June to reach a comprehensive agreement that would ultimately rescind tough western sanctions on Iran in exchange for its verifiable move away from the development of a nuclear bomb.

White House official Josh Earnest said Thursday that even the detention of three Americans in Iran and the unresolved case of a fourth missing in the country would not be a barrier to a possible nuclear deal.