SEOUL, South Korea (AP) - North Korea accused the South of sending patrol boats into its waters near a disputed western sea border and warned that any war moves would be "shattered to smithereens." The U.S. president said North Korea’s recent nuclear blast and other behavior tested the limits of diplomacy.

The threat Saturday by the communist government’s National Peace Committee came as many worried that a sea clash could quickly escalate into a major confrontation amid soaring tensions over the North’s recent nuclear and missile tests. Deadly battles occurred in the area in 1999 and 2002.

Without providing specifics, the North’s official Korean Central News Agency alleged that the patrol boats were crossing into its territorial waters on daily missions. It said such aggressors would be dealt "merciless punishment that will be beyond imagination," and that "war moves will be shattered to smithereens."

South Korea rejected the North’s allegation. Just two days earlier, the South accused the North of sending a patrol boat across its sea border in the same area. The boat turned back without incident after a 50-minute standoff with the South’s naval ships, the military said.

Earlier Saturday, South Korean President Lee Myung-bak said in a nationally televised Memorial Day speech that Seoul would not be cowed by the North’s nuclear and missile threats.

"I would like to make it clear that there will be no compromise with things that threaten our people and security," he said.

On a visit to France, President Barack Obama said the North’s nuclear test and missile launches were provocative. He said Washington was running out of diplomatic patience and suggested it would adopt a stronger response toward the North.

"Diplomacy has to involve the other side engaging in a serious way, and we have not seen that reaction from North Korea," Obama said Saturday. "I don’t think there should be an assumption that we will simply continue down a path in which North Korea is constantly destabilizing the region and we continue to act in the same ways."