The United States and South Korea staged an extensive war exercise simulating storming North Korea’s beach defenses as North Korea threatened to “annihilate” its enemies Saturday.

More than 17,000 U.S. troops and 300,000 South Korean troops along with 130 Australian and 60 New Zealand troops joined in the amphibious exercise in the city of Pohang, South Korea. The drill is set to be followed by a much larger, eight-week series of annual joint military exercises between the U.S. and South Korea.

Tensions in the region have been high since North Korea conducted its fourth nuclear test in January and a rocket launch last month, triggering fresh U.N sanctions.

According to a U.S. military statement, the United Nations Command reportedly informed the Korean People's Army in North Korea about the "non-provocative nature" of the joint military exercises.

But North Korea has responded with a threat of a "preemptive nuclear strike" and said Saturday that the military exercises were "reckless" and its forces would "go over" the operation for preemptive retaliatory strikes at those involved in the drill, according to state-run Korean Central News Agency.

"The revolutionary armed forces of the DPRK [Democratic People's Republic of Korea] holding tightly the arms to annihilate the enemies with towering hatred for them are waiting for the dignified Supreme Command to issue an order to launch a preemptive strike of justice," North Korea’s military said, according to South Korea's Yonhap News Agency.

The U.S. general in charge of Saturday's amphibious assault exercises insisted the drills were not due to the recent rise in tensions. "We've been planning for this exercise for over a year," Brigadier General John Jansen of the U.S. Marines told CNN.

On Saturday, reports, which cited U.S. officials, said that North Korea lost contact with one of its submarines that has been missing for days off its east coast. U.S. intelligence officials told CNN that the missing vessel likely suffered a technical problem during an exercise and that there is no confirmation yet on whether it is adrift at sea or has sunk.