POHANG, South Korea (Reuters) - U.S. and South Korean troops staged a big amphibious landing exercise on Saturday, storming simulated North Korean beach defenses amid heightened tension and threats by the North to annihilate its enemies.

The landing and assault drills on South Korea’s east coast were part of eight weeks of joint exercises between the allies which the South has said are the largest ever. The North has denounced the exercises as “nuclear war moves” and threatened to respond with an all-out offensive.

Tension on the Korean peninsula has been high since the North conducted its fourth nuclear test in January and followed that with a long-range rocket launch last month, triggering new U.N. sanctions.

About 55 U.S. marine aircraft and 30 U.S. and South Korean ships, including the USS Bonhomme Richard and USS Boxer, which carry AV-8B Harrier attack jets and V-22 Osprey aircrafts, took part in the assault on beaches near Pohang city, the U.S. navy said.

“They will penetrate notional enemy beach defenses, establish a beach head, and rapidly transition forces and sustainment ashore,” the U.S. military based in South Korea said in a statement before the exercise.

The North’s military said it was prepared to counter the U.S. and South Korean forces “with an ultra-precision blitzkrieg strike of the Korean style”.

“The revolutionary armed forces of the DPRK 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,” it said in comments carried by the state KCNA news agency.

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DPRK stands for the Democratic People’s Republic of Korea.

CNN reported on Saturday that North Korea has been searching for one of its submarines that has been missing for days off its east coast.

The submarine may be adrift under the sea or have sunk, perhaps after a technical problem during an exercise, CNN quoted U.S. officials with intelligence of secret U.S. monitoring of the North’s activities as saying.

North Korea has said it is developing submarine-launched ballistic missiles although doubts about that were raised after Western experts said publicly released footage of tests appeared to be fake.

On Thursday, North Korean leader Kim Jong Un watched as his forces fired two short-range ballistic missiles into the sea. This month the North conducted drills with what it said were newly developed large caliber rocket launchers.

Kim has ordered the country to improve its nuclear attack capability by conducting more tests, in defiance of a U.N. Security Council resolution adopted last week in response to the isolated state’s latest nuclear test.

Kim also said his country had miniaturized nuclear warheads to mount on ballistic missiles, although the U.S. and South Korean governments have expressed doubts about that too.

The South Korean and U.S. militaries have said they had notified the North of “the non-provocative nature” of the exercises involving about 17,000 American troops and more than 300,000 South Koreans.

The United States has about 28,500 troops stationed in South Korea.