Updated

The top U.S. military officer said Monday that the danger of war or hostilities on the Korean peninsula is rising, as North Korea warned that cooperation between its southern neighbor and the United States could trigger a nuclear war.

Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, told troops in Baghdad that North Korea has upped the ante in its aggression against South Korea -- a pattern he said is tied to preparations for Kim Jong Il to pass power to his son.

The latest example of bellicose rhetoric came with North Korea warning of nuclear war, as South Korea planned live-fire naval drills to run Monday through Friday. Those drills follow a North Korea artillery attack on front-line Yeonpyeong Island that killed two South Korean marines and two civilians.

The Nov. 23 artillery barrage, the North's first assault to target a civilian area since the end of the 1950-53 Korean War, began after the North said South Korea first fired artillery toward its territorial waters. South Korea says it fired shells southward, not toward North Korea, as part of routine exercises.

After the attack, South Korea staged joint military drills with the United States and also pushed ahead with more artillery exercises, despite the North's warning that they would aggravate tension.

A South Korean Joint Chiefs of Staff officer tried to play down the significance of this week's drills, saying they are part of routine military exercises and would not occur near the disputed western Korean sea border where last month's attack took place. The officer, who spoke on condition of anonymity because of office policy, gave no further details.

North Korea, however, lashed out at Seoul, accusing South Korea of collaborating with the United States and Japan to ratchet up pressure on Pyongyang.

That cooperation "is nothing but treachery escalating the tension between the North and the South and bringing the dark clouds of a nuclear war to hang over the Korean peninsula," Pyongyang's main Rodong Sinmun newspaper said in a commentary carried by the North's official Korean Central News Agency.

North Korea has often issued similar threats during standoffs.

In a show of unity, top diplomats from South Korea, the United States and Japan met in Washington last week and said they would not resume negotiations aimed at persuading North Korea to abandon its nuclear weapons program until the country's behavior changes. Mullen visited South Korea last week and warned Pyongyang to stop its "belligerent, reckless behavior."

On Monday, South Korean and U.S. defense officials met in Seoul for one-day discussions on North Korea and other issues that are part of regular defense talks, according to Seoul's Defense Ministry.

The Associated Press contributed to this report.