North Korea warned of a nuclear strike against South Korea and the United States as the allies began their annual military drills Monday aimed at countering potential threats from Pyongyang. The reclusive nation said it would turn Seoul and Washington into “a heap of ashes” if the exercises show any sign of violence toward its territory.

The two-week-long drills, which involve tens of thousands of South Korean and U.S. forces, come at a time of heightened tensions following the defection of a London-based North Korean senior diplomat and the announcement of the deployment of an U.S.-made advanced anti-missile system in South Korea by the end of 2017.

South Korea and the U.S. “should bear in mind that if they show the slightest sign of aggression on (DPRK's) inviolable land, seas and air ..., it would turn the stronghold of provocation into a heap of ashes through Korean-style pre-emptive nuclear strike,” North Korea’s military said in a statement, carried by the state-run Korean Central News Agency.

The statement also added that the North’s “first-strike” units are ready to mount retaliatory strikes on South Korean and U.S. forces involved in the drills, dubbed Ulchi Freedom Guardian, involving 25,000 American troops and 50,000 South Korea soldiers.

“The situation on the Korean peninsula is so tense that a nuclear war may break out any moment,” the North Korean military said in the statement, adding that it is “waiting for a historic chance to blow up all strongholds of aggression and war and liberate the southern half of Korea.”

The South Korean government had reportedly warned over the weekend that the North could carry out some sort of provocative action during or after the annual military drills that come days after Seoul confirmed the defection of Thae Yong Ho, North Korea's deputy ambassador in London, last week.

Thae, who reportedly lived in the U.K. for 10 years with his wife and family and disappeared from his home in west London several weeks ago, was criticized by Pyongyang's state media as a “human scum” and a criminal who had been ordered to return to the country pending investigation for a series of alleged crimes like embezzling government funds, leaking state secrets and sexually assaulting a minor.

Monday’s warning came weeks after North Korea test-fired two ballistic missiles in an apparent response to the scheduled deployment of the THAAD missile defense system in South Korea’s southern county of Seongju. The U.S. military said at the time that one of the two missiles exploded right after the launch, while the other landed into the sea off the North’s east coast.