North Korea warned today that US-South Korean co-operation could bring nuclear war to the region.
The warning came 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 due to run until 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 November 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 said it fired shells southwards, not towards North Korea, as part of routine exercises.
After the attack South Korea staged joint military drills with the US 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 were part of routine military exercises and would not occur near the disputed western Korean sea border where last month's attack took place.
North Korea, however, lashed out at Seoul, accusing South Korea of collaborating with the US and Japan to step up pressure on Pyongyang.
That co-operation "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 stand-offs.
In a show of unity, top diplomats from South Korea, the US and Japan met in Washington last week and said they would not resume negotiations aimed at persuading North Korea to abandon its nuclear weapons programme until the country's behaviour changed.
Admiral Mike Mullen, chairman of the US Joint Chiefs of Staff, visited South Korea last week and warned Pyongyang to stop its "belligerent, reckless behaviour."
Meanwhile South Korean and US defence officials met in Seoul today for one-day discussions on North Korea and other issues that are part of regular defence talks, according to Seoul's Defence Ministry.
At the opening of the meeting, US deputy assistant secretary of defence 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 due 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 US officials accompanying Steinberg will travel on to Seoul and Tokyo, Japan.
New Mexico governor Bill Richardson, meanwhile, leaves the US for North Korea tomorrow. Mr 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.