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U.S. and South Korean troops staged a big amphibious landing exercise Saturday, storming simulated North Korean beach defenses amid and threats by Pyongyang 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.

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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.

US and South Korean marines participate in an annual joint drill in Pohang, southeast of Seoul, South Korea. JEON HEON-KYUN / EPA

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".

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.

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.