SEOUL, South Korea (AP) - North Korea warned South Korea that any attempt to bring down the communist country would draw "strong measures" from its military, a threat issued Monday even as Pyongyang embarked on a flurry of diplomacy with Seoul, Washington and Beijing.

Pyongyang is poised to mobilize troops to defend itself, including a "world-level ultramodern striking force" that has not yet been publicly revealed, North Korea's Ministry of People's Security and the Ministry of State Security said in a statement.

North Korea will take "all-out strong measures to foil the treacherous, anti-reunification and anti-peace moves of the riff-raffs to bring down the dignified socialist system ... and destabilize it," said the statement carried by the official Korean Central News Agency.

The warning, stern but milder than threats made last year, was carefully timed to show tensions could flare if North Korea doesn't get what it wants from the round of diplomacy, said Jeung Young-tae, a North Korea expert at the state-run Korea Institute of National Unification in Seoul.

"They are using it as a negotiating card," he said.

The threat was issued as senior Chinese envoy Wang Jiarui met in Pyongyang with Choe Thae Bok, a high-level official in North Korea's ruling Workers' Party, amid an international push to persuade North Korea to return to stalled nuclear disarmament talks.

Footage broadcast by APTN in Pyongyang showed Wang visiting a modern new apartment and touring a fruit farm.

Wang told Choe that China, North Korea's longtime ally and benefactor, was ready to work with North Korea to boost bilateral ties, according to the Xinhua News Agency. The report did not mention the nuclear issue.

The envoy was expected to meet with North Korean leader Kim Jong Il later Monday to discuss the nuclear talks, South Korean cable network YTN said, without citing its source. Wang will likely bring Kim a letter from Chinese President Hu Jintao, the Chosun Ilbo newspaper said in a similar report.

Meanwhile, U.N. political chief B. Lynn Pascoe was due in Pyongyang on Tuesday, reportedly bearing a letter from U.N. Secretary-General Ban Ki-moon of South Korea. Pascoe's trip will be the first by a high-level U.N. official since 2004, according to Seoul's Foreign Ministry.

Pascoe will urge North Korea to rejoin the nuclear talks, and discuss the country's relationship with the world body, a U.N. official in New York said on condition of anonymity, citing policy.

North Korea walked away from the disarmament talks last year in anger over international condemnation of a long-range rocket launch. The regime later conducted a nuclear test, test-launched ballistic missiles and restarted its plutonium-producing facility, inviting widespread condemnation and tighter U.N. sanctions.

North Korea has reached out to the international community in recent months - an about-face that analysts say shows the regime is feeling pinched by sanctions. However, North Korea has made clear it wants sanctions lifted and a peace treaty to formally end the 1950-1953 Korean War before returning to the disarmament talks.

Pyongyang on Saturday released an American missionary detained since Christmas for illegal entry, and on Monday officials from the two Koreas met in a North Korean border town to discuss restarting joint tours suspended in 2008.

Monday's warning - citing South Korea's reported contingency plan for unrest in the North, propaganda leaflets critical of Pyongyang's communist system and military movements along the Koreas' disputed western sea border - shows security remains fragile even as relations improve, Jeung said.