Any new economic sanctions that Congress tries to impose against Iran during international negotiations to limit the Islamic republic's nuclear program will be vetoed, President Barack Obama warned lawmakers on Tuesday, even as he acknowledged the ongoing talks are difficult and may not succeed.

It was the sharpest part of a State of the Union speech that overall was skimpy on details for his foreign policy agenda for the year.

Obama also briefly re-affirmed his promises to end the war in Afghanistan this year, pursue al-Qaida with limited strikes and support Syrian opposition groups in their bloody three-year quest to push their ruler from power. But he avoided clarifying some sticky key issues, including how many — if any — troops might remain in Afghanistan beyond December, or how he will try to convince Americans and foreigners alike that the National Security Agency is not snooping into their telephone and Internet records.

Faced with a Mideast awash with strife — from Cairo to Jerusalem to Beirut to Damascus to Baghdad — Obama focused on reversing a generation-old feud with Iran as a potential bright spot in the region.

"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," Obama said.

The president said he's "cleared-eyed" about longstanding mistrust between Iran and six world powers that are working to prevent Tehran 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.

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

Iran long has 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. Obama also said he "will be the first to call for more sanctions" if Iran reneges on the deal.

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.

"The American people — Democrats and Republicans alike — overwhelmingly want Iran held accountable during any negotiations," Sen. Mark Kirk, R-Ill., said after Obama concluded his speech.

Obama also took credit for forcing the start of dismantling Syria's chemical weapons stockpile — a process he said was pushed by American diplomacy and threat of force.

The Obama administration had threatened to strike Syria's government, but backed down, after an Aug. 21 chemical weapons attack outside Damascus that the U.S. said killed more than 1,400 people. A month later, the U.N. Security Council approved a resolution to eliminate Syria's chemical weapons arsenal.

Obama promised anew to support opposition groups that are fighting to overthrow Syrian President Bashar Assad in that country's three-year civil war, and said the Syrian people deserve a future that is free of dictatorship, terror and fear.