North Korea (search) claimed Monday that it has bolstered its nuclear arsenal to prevent an invasion, the North's official broadcaster said.

"We have taken a serious measure by increasing nuclear arms arsenal in preparation for any invasion by enemies," the North's Korean Central Broadcasting Station said in a commentary, according to a report by South Korea's Yonhap news agency.

The commentary said ongoing joint U.S.-South Korean military exercises were "preparation for war against us."

The North has frequently claimed it would increase its nuclear deterrent in response to the perceived threat of invasion by the United States, but the Monday announcement appeared to be the first time Pyongyang (search) has claimed to actually do so. However, the North's statement didn't elaborate on how its arsenal was increased. The United States has repeatedly denied it intends to attack North Korea.

Last month, the North's government said for the first time that it had nuclear weapons and would indefinitely boycott the six-nation talks that began in 2003 to resolve the crisis.

Monday's new claim came after Secretary of State Condoleezza Rice (search) left the region following visits to Japan, South Korea and China to seek a way to convince the North to return to stalled six-nation disarmament talks, which also include Russia.

Meanwhile, North Korean Prime Minister Pak Pong Ju left for a visit to China, the North's official media said. Washington has been looking to Beijing to increase pressure on its communist ally to return to nuclear disarmament talks.

There were no further details in the brief report by the North's Korean Central News Agency on Pak's departure, other than that the "official goodwill visit" was at the invitation of Chinese Premier Wen Jiabao.

However, it was expected that Chinese officials would discuss the recent visit by Rice and attempts to lure the North back to six-nation nuclear disarmament talks.

Rice suggested during her trip that time is running out for the North to return to the negotiations, and that Washington will pursue other means — assumed to include U.N. Security Council sanctions — if Pyongyang refuses to return to the bargaining table. The United States has said it has no intention to attack the North.

China is the North's closest ally and a key benefactor providing needed energy aid and other assistance.