WASHINGTON — In a show of unity, President Barack Obama and South Korean President Lee Myung-bak have issued a stern warning to North Korea, vowing to stop the reclusive nation from using nuclear brinkmanship to squeeze concessions from a frightened world.

Lee, a conservative former businessman whom Pyongyang regularly pillories as a "traitor," told reporters Tuesday in the White House Rose Garden that the world will not respond to North Korea's recent nuclear and missile tests and its threats of nuclear war by offering financial incentives.

"They will not be able to gain compensation by provoking a crisis," Lee said. "This has been a pattern in the past, but it will no longer be."

Lee responded to a question about a possible North Korean attack by saying his country's strong alliance with the United States will force the North to "think twice about taking any measures that they will regret."

Lee said he and Obama agreed that "under no circumstance are we going to allow North Korea to possess nuclear weapons." North Korea already has tested two underground nuclear devices and is believed by U.S. intelligence to possess material to make several nuclear bombs.

Obama also talked tough, promising to break the cycle of allowing Pyongyang to create crises and then reap rewards when it backs down.

Despite the display of unity on nuclear matters, the leaders seemed far apart on an ambitious free trade agreement that is currently stalled over U.S. lawmakers' worries that it could hurt an already struggling American auto industry.

The main focus of the presidents' meeting, however, was North Korean rhetoric that has become increasingly heated in the wake of a United Nations rebuke of the North's second nuclear test on May 25.

North Korea is furious over U.N. sanctions that toughen an arms embargo and authorize ship searches in an attempt to thwart the North's nuclear and ballistic missile programs. The United Nations, however, did not authorize military force to compel the measures.

Obama said the leaders agreed that the new U.N. resolution must be fully enforced, something the North has said it would consider an act of war. The North has responded to the U.N. sanctions by promising to weaponize all its plutonium and step up its nuclear bomb-making by enriching uranium.

Today, the government-run Minju Joson newspaper published a commentary saying, "If the U.S. and its followers infringe upon our republic's sovereignty even a bit, our military and people will launch a one hundred- or one thousand-fold retaliation with merciless military strikes."

U.S. officials have said the North Koreans also appear to be making preparations for a third nuclear test.