Washington (CNN) President Barack Obama promised Friday to veto a plan by members of Congress to issue additional sanctions on Iran while diplomatic negotiations regarding Iran's nuclear plan are under way.

"I will veto a bill that comes to my desk and I will make this argument to the American people as to why I'm doing so," Obama said at a joint press conference with United Kingdom Prime Minister David Cameron, referencing a plan supported by some in his own party to increase sanctions on Iran through Congress before a deal is reached by international negotiators.

"My main message to Congress at this point is just hold your fire. Nobody around the world least of all the Iranians doubt my ability to get additional sanctions pass if these negotiations fail," Obama added later.

Cameron said he was also calling American senators and urging them not to pass additional sanctions.

The wide ranging joint press conference covered issues as global as the terror attacks in France to as Beltway as what Obama thinks about his one time opponent Mitt Romney considering another presidential bid ("No comment," Obama said while smiling).

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