WASHINGTON -- North Korea's planned long-range rocket launch threatens to wreck its recent food-for-nuclear concessions deal with the United States and dim hopes for better relations under new leader Kim Jong Un.

The North's announcement Friday marked a sharp and sudden turn 17 days after the two countries offered unexpected signs of optimism that three years of tensions were easing. Such a launch would violate a UN ban.

"It's a real slap in the face," said Victor Cha, a White House director for Asia policy under President George W. Bush. "It undercuts a lot of theories that the young leadership might be different. If anything, it shows that it's very much the same as before, only more unpredictable."

It is an embarrassment in an election year for President Barack Obama, who has been labeled by Republican presidential candidates as naive in his foreign policy. Republican lawmakers have accused his administration of "appeasing" North Korea by offering 240,000 tons of food in exchange for a freeze on nuclear activities and a freeze on nuclear and long-range missile tests.

If North Korea carries out the launch, it will be hard to keep alive the accord announced Feb. 29.

State Department spokeswoman Victoria Nuland said a satellite launch would be a "deal-breaker" and indicated that the United States would be unlikely to send the food.

A launch in violation of the North's commitments, she said, would undermine confidence that the North would allow proper monitoring of the distribution of the aid.

The development shows the pitfalls of negotiating with a secretive government, which views its nuclear program as a deterrent against invasion. The United States retains 28,000 troops in South Korea, a legacy of the 1950-53 Korean War. The Koreas remain in a state of war because the war ended without a peace treaty.

Previous U.S. efforts over the past two decades to persuade North Korea to disarm have ended in disappointment. Even before Friday's announcement, a group of five Republican senators wrote to Secretary of State Hillary Rodham Clinton accusing the administration of accepting the North's "hollow commitments."

North Korea says the rocket launch would be for peaceful means. But the same kind of technology is used for ballistic missiles, which could eventually provide a delivery system for a nuclear weapon if the North becomes able to miniaturize one for use on a warhead.

An international nuclear summit March 26-27 in South Korea, to be attended by Obama, will provide a high-profile opportunity to crank up diplomatic pressure on the North over its plans to conduct the rocket launch.

"The U.S. will probably really lean on the Chinese," said Jonathan Pollack, an expert on North Korea's nuclear program at the Brookings Institution think tank. He said he expected the message to China to be: "You remember what happened last time they tried to launch a satellite?"

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