President Barack Obama delivers the State of Union address before a joint session of Congress in the House chamber Tuesday, Jan. 28, 2014, in Washington. (AP Photo/Larry Downing, Pool)

WASHINGTON - Negotiations to limit Iran's nuclear program will be difficult and may not succeed, President Barack Obama said Tuesday, but he warned Congress that any new economic sanctions against Tehran while the discussions are ongoing will be vetoed.

In his annual State of the Union speech, Obama said he's "cleared-eyed" about longstanding mistrust between Iran and six world powers that are working to prevent the Islamic republic from enriching enough uranium to build nuclear weapons. He also credited the U.S. for what he described as leading the way toward an interim agreement that has all but frozen Iran's nuclear program for the first time in a decade.

"The sanctions that we put in place helped make this opportunity possible," Obama said. "But let me be clear: If this Congress sends me a new sanctions bill now that threatens to derail these talks, I will veto it."

"For the sake of our national security, we must give diplomacy a chance to succeed," he said.

Iran has long maintained that its nuclear program is only for medical and peaceful energy programs.

But after years of negotiating, Iran agreed in November to slow its uranium enrichment program to a level that is far below what would be necessary to make a nuclear bomb. It also agreed to increased international inspections to give world leaders confidence that it is not trying to build weapons in secret.

In exchange, the U.S. and five other nations — Britain, Germany, France, Russia and China — agreed to ease an estimated $7 billion worth of international sanctions against Iran's crippled economy for a six-month period while negotiators try to broker a final settlement.

But critics in Congress want sanctions to remain in place, claiming that their harsh economic impact is what forced Iran to the negotiating table in the first place.

Obama pushed back in his Tuesday speech, and said he "will be the first to call for more sanctions" if Iran reneges on the deal.

"But if Iran's leaders do seize the chance, then Iran could take an important step to rejoin the community of nations, and we will have resolved one of the leading security challenges of our time without the risks of war."

News from © The Associated Press, 2014