Pablo Martinez Monsivais/AP Photo

ABC News' Jake Tapper and Mary Bruce report:

SEOUL, Republic of Korea -Despite threats from North Korea that any criticism of its nuclear weapons program would be considered "a declaration of war," President Obama and South Korean President Lee Myung-bak today presented a united front to warn North Korea's leaders that if they go forward with plans for a long-range missile test next month, their people will ultimately be the ones who suffer.

Discussing a package of promised nutritional aid for the malnourished and impoverished North Korean people, President Obama said his administration had indicated to the leaders of North Korea "very directly" that "it would be difficult to move forward with that package if they show themselves unable to make commitments that they've made, even a month earlier."

Lee said that he and Mr. Obama "agreed to respond sternly to any provocations and threats by the North and to continually enhance the firm South Korea-US defense readiness."

"North Korea will achieve nothing by threats or by provocations," President Obama said. "North Korea knows its obligations and it must take irreversible steps to meet those obligations."

The missile launch is scheduled to coincide with the 100th anniversary of the birth of Kim Il-sung, the founder of North Korea and grandfather of the current leader of the country, Kim Jong-un. President Obama even said it was "not clear exactly who is calling the shots" in North Korea.

President Lee suggested that the missile launch would further alienate the people of North Korea from its leadership, saying that "from the perspective of the people of North Korea, they will understand that its leaders are spending hundreds of millions of dollars just to test launch a long-range missile and, watching their leader do so, they are not going to feel proud that their country was able to launch a long-range missile, rather they will get, truly understand the nature of their leaders and understand why they have to go through such hardships."

Asked for his views of Kim Jong-un, Mr. Obama said "t's not clear exactly who is calling the shots and what their long-term objectives are."

President Obama said such a missile launch would not only violate the country's own commitments and international obligations, "it would only deepen North Korea's isolation, damage further its relations with its neighbors and serious undermine the prospects of future negotiations…They need to understand that bad behavior will not be rewarded. There has been a pattern for decades, in which North Korea thought that if they acted provocatively then somehow they would be bribed into ceasing and desisting acting provocatively and President Lee and I have agreed from the start in our relationship that we're going to break that pattern."

President Obama is scheduled to also meet with Chinese president Hu Jintao tomorrow, and today he had a harsh assessment of China's ability to influence North Korea.

"My suggestion to China is how they communicate their concerns to North Korea should probably reflect the fact that the approach they've taken over the last several decades hasn't led to a fundamental shift in North Korea's behavior," he said.

-Jake Tapper and Mary Bruce