WASHINGTON – New congressional action against Iran would blow up an international diplomatic effort to stop their nuclear program through peaceful means, US President Barack Obama said on Friday, in a pointed warning to lawmakers on Capitol Hill.Joined by British Prime Minister David Cameron at the White House, Obama said he would veto any new legislation that would “undercut,” “undermine” and “set back” the nuclear talks, now a year old and facing a deadline in March for diplomats to reach a political agreement.Leaders in Congress, now united under Republican control, are preparing a bill that would trigger new sanctions against Iran should the negotiations fail. The working draft of the bill, they argue, does not impose new sanctions during the talks.But the Iranians wouldn’t interpret the bill that way, Obama said. The result, he said, would be a collapse in talks with the United States to blame, and the chances of a “military showdown” suddenly amplified.“There would be some sympathy to that view around the world, which means that the sanctions that we have in place now would potentially fray,” Obama added.“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,” he added. “And Congress will have to own that, as well.”The White House seeks a deal that would cap and roll back key aspects of Iran’s nuclear program, if not fully dismantle the vast infrastructure currently in place. From a strategic standpoint, the administration seeks to place the onus of a diplomatic failure on the Iranians – a strategy compromised by congressional action.Cameron said he respected the sovereignty of Congress and would not lobby lawmakers, answering a reporter’s question on whether he had made individual calls to US senators on the matter.But he also made clear that the United Kingdom opposes any new sanctions measure from Washington.The talks require “the space for negotiations to succeed,” he said, adding that the timing was wrong for new sanctions measures.Obama, once again, put the chances of success at the negotiating table at “less than 50-50,” and said, “if Iran cannot say yes...then we’re going to have to explore other options.”Sanctions are “not the only option,” he continued, in a series of references to the prospect of force.“I will veto a bill that comes to my desk,” he said, asking lawmakers “to hold off for a few months to see if we have the possibility of solving a big problem without resorting, potentially, to war. “If I’m not persuading Congress,” he added, “I promise you, I’ll be taking my case to the American people on this.”Answering a follow-up question from a reporter on the matter, he tampered down his rhetoric on the potential for military confrontation.“I am not, repeat not, suggesting we are on an immediate war footing should negotiations with Iran fail,” he said.