SEOUL, South Korea (AP) — North Korea warned Monday that U.S.-South Korean cooperation could bring a nuclear war to the region, as the South began artillery drills amid lingering tension nearly three weeks after the North’s deadly shelling of a South Korean island.

The South’s naval live-fire drills are scheduled to run Monday through Friday at 27 sites. The regularly scheduled exercises are getting special attention following a North Korean 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. Adm. Mike Mullen, chairman of the U.S. Joint Chiefs of Staff, 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.

At the opening of the meeting, U.S. Deputy Assistant Secretary of Defense Michael Schiffer said “the United States stands shoulder to shoulder with the Republic of Korea and with the Korean people in the face of recent North Korean provocations,” referring to South Korea by its formal name.

Deputy Secretary of State James Steinberg was also set to visit China later this week for talks on North Korea amid international pressure for Beijing to use its diplomatic clout to rein in North Korea, its ally. After the China meeting, senior U.S. officials accompanying Steinberg will travel on to Seoul and Tokyo.

New Mexico Gov. Bill Richardson, meanwhile, leaves the United States for North Korea on Tuesday. Richardson, who has often acted as a diplomatic troubleshooter, has made regular visits to North Korea and has also hosted North Korean officials in New Mexico.