PYONGYANG, North Korea (AP) — North Korea promised Monday to reduce South Korea's conservative government "to ashes" in less than four minutes, in an unusually specific escalation of recent threats aimed at its southern rival.

The statement by North Korea's military, carried by state media, comes amid rising tensions on the Korean peninsula. Both Koreas recently unveiled new missiles, and the North tried unsuccessfully to launch a long-range rocket earlier this month.

The growing animosity has prompted worries that North Korea may conduct a nuclear test — something it did after rocket launches in 2006 and 2009. South Korean intelligence officials say recent satellite images show the North has been digging a new tunnel in what appears to be preparation for a third atomic test.

North Korea's military vowed in its statement to begin "special actions" soon against the government and conservative media companies that would "reduce all the rat-like groups and the bases for provocations to ashes in three or four minutes, (or) in much shorter time, by unprecedented peculiar means and methods of our own style."

North Korea regularly criticizes Seoul and just last week renewed its promise to wage a "sacred war," saying South Korean President Lee Myung-bak had insulted the North's April 15 celebrations of the birth centennial of national founder Kim Il Sung.

But Monday's message, distributed by the state-run Korean Central News Agency and attributed to the Korean People's Army's Supreme Command, was unusual in promising something soon and in describing a specific period of time.

Seoul expressed worry that the threats were hurting relations between the countries and increasing animosity.

"We urge North Korea to immediately stop this practice," Unification Ministry spokesman Kim Hyung-suk told reporters, according to the ministry. "We express deep concern that the North's threats and accusations have worsened inter-Korean ties and heightened tensions."

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A Defense Ministry official, speaking on condition of anonymity, citing office rules, said no special military movement had been observed in the North.

Some South Korean analysts speculated the North's statement was meant to unnerve Seoul, while others said the North could be planning terrorist attacks.

It seemed unlikely that North Korea would launch a large-scale military attack against Seoul, which is backed by nearly 30,000 U.S. troops stationed in the South, said Kim Young-soo, a professor at Sogang University in Seoul.

The threat follows U.N. condemnation of the North Korean launch of a long-range rocket that exploded shortly after liftoff on April 13. Washington, Seoul and others called the launch a cover for testing long-range missile technology. North Korea said the launch was meant to put a satellite into orbit.

Relations between the Koreas have been abysmal since Lee took office in 2008 with a hard-line policy that ended unconditional aid shipments to the North.

In Beijing, North Korea's biggest ally, China's top foreign policy official met Sunday with a North Korean delegation and expressed confidence in the country's new young leader, Kim Jong Un.

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Associated Press writers Youkyung Lee and Jiyoung Won in Seoul, South Korea, contributed to this report.