The announcement on North Korean official news agency KCNA came as South Korea readied for firing drills on Yeonpyeong island near a disputed maritime border with the North for the first time since November’s exchange of artillery fire.

“The strike will play out a more serious situation than on Nov. 23 in terms of the strength and scope of the strike,” KCNA said.

A leading South Korean defense analyst said he doubted the North would carry out its threat, which rattled financial markets, and South Korea’s Defense Ministry said the exercise planned for Dec. 18-21 would go ahead.

The North had said its November shelling was a response to South Korean “provocations” after an artillery battery on the island fired in what Seoul said was a routine drill.

North Korea’s warning came after Seoul promised a more robust response to any further attacks on its territory. The shelling of the island was the first time since the Korean war that the North had attacked South Korean territory.

“They would have to be committing to a full-out war if they did that (struck again),” said Baek Seung-joo of the Korea Institute for Defense Analyzes, an expert on North Korea’s military strategy.

“What’s likely is they will do something as a face-saving action, such as firing their own artillery near the disputed waters,” he said.

China, the North’s main backer, has said that Pyongyang had promised restraint. The threat of a new attack by the North came as China told visiting US Deputy Secretary of State James Steinberg the two big powers should cooperate more in defusing tension on the Korean peninsula.

It also came as US diplomatic troubleshooter Bill Richardson visited Pyongyang in an effort to “reduce the tension on the Korean peninsula.”

China’s top diplomat, Dai Bingguo, urged closer coordination over the Korean peninsula during talks with Steinberg, the second most senior official in the US State Department, the state-run Xinhua news agency reported on Friday.

Steinberg was in Beijing for three days till Friday to press China to rein in North Korea, which as well as shelling the island last month also disclosed advances in uranium enrichment which could give it a new path to make nuclear weapons.

China has avoided publicly condemning its long-time ally over the shelling and nuclear program, and instead pleaded with other powers to embrace fresh talks with North Korea.

Russia summoned the ambassadors of South Korea and the United States to express “extreme concern” and urged the two countries to halt the exercise to prevent “the further escalation of tensions,” the Russian the Foreign Ministry said.

North Korea’s threatened strike came as is sought the restart of six-party nuclear talks with the United States, China, South Korea, Japan and Russia.

It wants the talks to resume without preconditions, something Washington and South Korea have ruled out as they do not want to reward Pyongyang for hostile actions.