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Advanced U.S. F-22 stealth jet fighters were poised in South Korea for war games Monday as North Korea said its nuclear-weapons program was non-negotiable.

The U.S. military command in South Korea said it flew the single-seat, twin-engine fighter aircraft, known as the Raptor, to Osan Air Base, 40 miles south of Seoul, from Japan's Kadena Air Base near Okinawa, to showcase its most potent weaponry to North Korea.

The jets -- among the most expensive and advanced in the U.S. Air Force's arsenal -- have never been used in combat, but would likely be the first aircraft used in a conflict with North Korea, The Wall Street Journal reported.

The super-maneuverable fighters -- which can evade radar and air-defense systems -- could be sent in to take out air-defense missiles and radars before bombers target missile launch sites or other targets, the newspaper said.

They also could be used to escort nuclear-capable B-2 stealth bombers if those jet fighters, designed to penetrate dense anti-aircraft defenses, are used in a strike, it said.

Use of the F-22 in the military training exercises shows that "despite challenges with fiscal constraints," U.S. forces "are battle-ready and trained to employ air power to deter aggression, defend [South Korea] and defeat any attack against the alliance," the U.S. military command said in a statement.

North Korea "will achieve nothing by threats or provocations, which will only further isolate North Korea and undermine international efforts to ensure peace and stability in Northeast Asia," the statement said.

North Korean leader Kim Jong Un, meanwhile, was quoted by the official Korean Central News Agency as saying he was unfazed by U.S. threats.

"The enemies are using both blackmail, telling us that we cannot achieve economic development unless we give up nuclear weapons, and appeasement, saying that they will help us live well if we choose a different path," Kim was quoted as saying Sunday during the plenary meeting of the Central Committee of the ruling Workers' Party.

But North Korea must expand its nuclear arsenal both "in quality and quantity, as long as the United States' nuclear threat continues," he said.

He said his country's nuclear weapons were "neither a political bargaining chip nor a thing for economic dealings."

President Barack Obama and National Security Adviser Thomas E. Donilon have urged Kim to learn from Myanmar, where political changes have led to billions in debt forgiveness, sweeping development assistance and a an influx of foreign investment.

They said North Korea would face more sanctions and deeper isolation if it continues on its current path.