North Korea threatened South Korea and the United States on Tuesday with "merciless retaliation" over their ongoing joint military drills.

Pyongyang warned both countries they "should take the whole responsibility for catastrophic results from (a) reckless war game," according to comments by the North's military published by the state-run Korean Central News Agency (KCNA).

South Korean and U.S. troops began 10 days of joint military exercises on Monday. About 50,000 South Korean forces and 17,500 U.S. military are taking part.

Pyongyang views the annual drills as an invasion rehearsal. South Korea and the U.S. describe them as defensive in nature. This year's drills come amid a war of words between North Korea and the U.S. over the former's threat to launch missiles into waters near the U.S. territory of Guam, although tensions have eased in recent days.

Adm. Harry Harris, the commander of U.S. forces in the Pacific, said the drills are critical for the allies to maintain readiness against an aggressive North Korea.

"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," Harris said during a news conference at the Osan Air Base in South Korea.

Vincent Brooks, commander of U.S. Forces Korea, said the allies should continue the war games until they "have reason not to" and that reason "has not yet emerged."

In a separate opinion piece published by KCNA, North Korea described President Trump as a leader who tweets "weird articles of his ego-driven thoughts" and "spouts rubbish" that make life for his aides difficult. In previous commentaries, the North has described Trump as "going senile" and a "war maniac bereft of reason."

A North Korean diplomat appearing at a United Nations forum on nuclear disarmament in Geneva on Tuesday said Pyongyang would not negotiate over its nuclear program. Ju Yong-chol said his country's nuclear deterrence was a "justifiable" and "legitimate" form of self-defense in the face of hostile U.S. foreign policy.

Trump has said using military force against North Korea is an option to prevent it from mastering the technology needed to deliver a nuclear weapon to the U.S. via a missile. North Korea has conducted five successful nuclear tests but security experts disagree over whether it has the ability to deploy nuclear material on a long-range missile.

In an attempt to further isolate North Korea, the U.S. Treasury Department imposed fresh sanctions on 16 Chinese and Russian individuals and entities Tuesday.

The move is designed to crack down on groups and businesses that could be helping Pyongyang develop its missile and nuclear programs.