US President Barack Obama on Friday urged Congress not to impose new sanctions on Iran over its disputed nuclear program, threatening to veto any such legislation that lands on his desk.

Obama told a joint press conference with British Prime Minister David Cameron that Iran was already chafing under existing sanctions and had not accelerated its program, and that he would strongly urge Congress not to torpedo the ongoing talks with Tehran.

"Congress needs to show patience," Obama said.

"We'll see how persuasive I am. But if I'm not persuading Congress, I promise you, I'm going to be taking my case to the American people on this," he warned.

Obama has faced mounting calls from Republican critics for tougher new sanctions on Iran, with lawmakers saying a debate on more stringent measures could take place in the US Senate within weeks.

But new sanctions would "jeopardize the possibility of... providing a diplomatic solution to one of the most difficult and long-lasting national security problems that we've faced in a very long time," Obama said.

"I will veto a bill that comes to my desk."

Cameron also spoke out against calls for further sanctions on Iran, saying negotiations needed "space" to succeed.

"We remain absolutely committed to ensuring that Iran cannot develop a nuclear weapon," Cameron said.

"The best way to achieve that now is to create the space for negotiations to succeed. We should not impose further sanctions now."

Iran and major world powers have given themselves until late June to reach a comprehensive agreement that would prevent Tehran from developing a nuclear bomb, a goal it denies having, in return for an easing of punishing economic sanctions.

A flurry of talks have been held this week, including meetings in Paris on Friday between Iranian Foreign Minister Mohammad Javad Zarif and his French and US counterparts.

Both sides have remained relatively tight-lipped about whether any progress is being made, but France said Friday that "significant" questions must be answered before a deal can be struck.

Sunday will see fresh talks in Geneva between Iran and the so-called P5+1 group -- the United States, Britain, China, France, Germany and Russia -- seeking to break a stalemate that has seen two earlier deadlines pass without an accord.

- Two paths in Congress -

However, Republicans in the United States are seeking to shape US policy on Iran by two paths being crafted in Congress.

One tactic envisages adoption of a bill requiring Obama to submit any nuclear accord reached with Iran to Congress for approval.

Another strategy is based on deferred economic sanctions that would be activated if Tehran refuses to sign a final deal or is seen as violating its terms.

A top US senator said debate could begin at the end of January or in early February.

Obama administration officials are vehemently opposed to any new measures.

"I've always said that the chances that we can actually get a diplomatic deal are probably less than 50/50. Iran is a regime that is deeply suspicious of the West," Obama said.

"But if, in fact, we still have an opportunity to get a diplomatic deal that provides us verifiable assurances that they are not developing a nuclear weapon, that is the best possible outcome that we can arrive at right now."

Washington's UN envoy Samantha Power warned Monday that imposing new sanctions "will almost certainly" see to the end of negotiations.