WASHINGTON, Jan. 16 -- U.S. President Barack Obama and visiting British Prime Minister David Cameron warned on Friday that extra sanctions on Iran would derail the ongoing efforts to prevent Tehran from getting nuclear weapons.

"We agree that the international community needs to remain united as we seek a comprehensive, diplomatic solution to prevent Iran from obtaining nuclear weapons," said Obama at a joint press conference in the White House with Cameron. "Additional sanctions on Iran at this time would undermine that international unity and set back our chances for a diplomatic solution."

Calling the current engagement with Iran "a very hard-nosed discussion" of Iranian nuclear program, Obama said the passing of the new sanction bill, which recently pit the White House against Congress, would be used by the Iranians to blame the United States for "operating in bad faith" and to step away from the current talks.

The failure to reach a nuclear deal due to Congress sanction bill may lead to military alternatives, warned a stern-faced Obama,

"Congress should be aware that if this diplomatic solution fails, then the risks and likelihood that this ends up being at some point a military confrontation is heightened," said Obama. " Congress will have to own that as well."

Washington's reluctance to pose new sanctions on Iran was echoed by Cameron, who was on a two-day state visit to the United States.

Calling the prospect of imposing extra sanctions " counterproductive", Cameron said the success of nuclear talks with Tehran required "space for negotiation."

"It could put at risk the valuable international unity that has been so crucial to our approach," he said

Admitting that he had contacted several U.S. senators personally to stress Downing Street's stance on the issue, Cameron explained his purpose as "to stand alongside America" on Iranian unclear issues.

However, while staunchly defending their opposition to any new sanctions, both leaders said the odds of a successful deal with Iran were less than 50-50.

As U.S. negotiators are meeting with their Iranian counterparts in a fresh round of nuclear talks, the White House and U.S. Congress are heading into a deadlock over whether to impose extra sanctions on Iran.

So far, U.S. lawmakers have already finished a new bipartisan bill on Iran sanctions and the Senate intends to vote on it well before current rounds of international nuclear talks end in June. Meanwhile, Obama threatened to veto any sanction bill, calling it a practice that "worth doing".

"Congress needs to show patience," said Obama at the joint press conference. "With respect to the veto, I said to my Democratic Caucus colleagues yesterday that I will veto a bill that comes to my desk."

The squabble over the new sanction bill was the latest of a series of issues which reflected hostility between the White House and a Republican-controlled Congress. In a closed-door session with Senate Democrats on Thursday, Obama reportedly promised to stand firm against Republican efforts to dismantle his agenda and urged his Democratic colleagues to help sustain his expected veto.

U.S. negotiators and their Iranian counterparts on Thursday began their three-day session of nuclear talks. The bilateral engagements would then be followed by talks on Jan. 18 between Iran and the P5+1 group (five permanent members of the UN Security Council plus Germany).

Negotiators have so far failed twice to reach a deadline on a comprehensive deal, under which Tehran would suspend its nuclear program while the West would loosen economic sanctions.