North Korea threatened “merciless retaliation” against US-South Korean military exercises on Tuesday, claiming they are laying the groundwork for an invasion.

“No one can vouch that these huge forces concentrated in South Korea will not go over to an actual war action now that the military tensions have reached an extreme pitch in the Korean Peninsula,” President Kim Jong Un’s government said in a statement, the Associated Press reported. “Moreover, high-ranking bosses of the U.S. imperialist aggressor forces flew into South Korea to hold a war confab. Such huddle is increasing the gravity of the situation.”

It went on to accuse the US military of deploying “lethal” weapons during the war games that involve “a beheading operation” intended to remove Kim from power.

But American officials said the drills, which began on Monday, are needed to maintain readiness against the bellicose reclusive regime that has vowed to strike the US mainland with nuclear weapons.

“A strong diplomatic effort backed by a strong military effort is key because credible combat power should be in support of diplomacy and not the other way around,” Adm. Harry Harris, the commander of the US forces in the Pacific, said during a news conference at the Osan Air Base in South Korea.

Tensions between the US and North Korea have risen over the past several months as Kim’s military continues to test-fire rockets that could reach the US.

The United Nations Security Council earlier this month imposed severe economic sanctions on Pyongyang for continuing work on its weapons systems.