SEOUL, South Korea — The U.S., Japan and South Korea began joint exercises Monday to simulate how they would target Kim Jong Un's ballistic missiles in the event of an attack, military officials from South Korea and Japan said.

North Korea last week said the outbreak of war was "an established fact" following another set of U.S.-South Korea exercises — although the isolated country has often issued such fiery rhetoric without action.

Monday's drills involved two U.S. destroyers, as well as one warship each from Japan and South Korea, a South Korean military spokesman told NBC News.

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The exercises saw the three countries sharing tactical information "during a simulation to target North Korean ballistic missiles," the spokesman said, adding that they came "in response to growing North Korea's nuclear and missile threats."

The drills were also confirmed to Reuters by Japan's Maritime Self-Defense Force.

Last month, North Korea conducted what it said was its most powerful intercontinental ballistic missile test to date, which it claimed could target anywhere in the U.S.

Any nuclear weapon capable of doing so, however, would also need carry a small, lightweight warhead and be able to withstand reentry into the Earth's atmosphere.

Experts are unsure exactly how close Kim is to achieving his goal, and many believe he wants the weapons to deter the U.S. from trying to topple his regime, rather than to strike them without provocation.

The North has offered to freeze its nuclear and missile programs in exchange for the U.S. stopping its joint drills with South Korea. Russia and China also back what the latter calls a "dual suspension" solution to the standoff.

Earlier this year, Secretary of State Rex Tillerson rejected such a freeze because he said it would effectively enshrine the nuclear capability that North Korea has already achieved and leave little leeway to dismantle its program as it stands, something that would be unacceptable to the U.S.

Stella Kim reported from Seoul. Alexander Smith reported from London.