President Obama on Friday vowed to veto any bill Congress sends him that could tighten economic sanctions on Iran, saying it would undermine negotiations seeking to limit the country's ability to build a nuclear weapon.

“My main message to Congress at this point is, just hold your fire,” Obama said in a joint White House news conference with British Prime Minister David Cameron.

If the negotiations fail, Obama said, then he would be open to considering tougher measures, with military action being a final option. "The bottom line is this: We may not get there, but have a chance to resolve the nuclear issue peacefully," the president said.

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Cameron, whose country is one of six world powers involved in the yearlong diplomatic efforts, acknowledged that he began calling U.S. senators Friday morning to make the same point—"not to tell Congress what it should do, but to make the point as a country that stands alongside America that it’s the opinion of the United Kingdom that the further threat of sanctions won't actually help to bring the talks to a successful conclusion."

The so-called P5+1 coalition has set a March 1 deadline to reach a general agreement with Iran, and a July 1 deadline to hammer out the details. The goal is to come up with a mechanism that would curtail Iran's ability to develop a nuclear weapon in exchange for the easing of economic sanctions. Iran has temporarily frozen its program and has rolled back some of its stockpile. But it appears the talks have stalled. Secretary of State John Kerry met with his Iranian counterpart, Foreign Minister Zarif, in Paris Friday after spending five hours with him Thursday in Geneva.

Friday's news conference was the first time Obama said he would veto the sanctions bill, even though the existing version would would only be triggered if the talks fail. Even that would be viewed as a deal-breaker by Iran, the president said.

He didn't mention that Iran has an incentive to stay at the table: They get partial sanctions relief and can sell some oil as long as they continue negotiating.

IN-DEPTH

— Andrea Mitchell and Jon Schuppe