North Korea said for the first time on Thursday that it had tested technology for launching several nuclear warheads with a single missile, days after President Vladimir V. Putin of Russia visited the North and raised the prospect of expanded military and technical cooperation.

The test on Wednesday was “aimed at securing the MIRV capability,” the North’s state-run Korean Central News Agency reported. MIRV stands for “multiple independently targetable re-entry vehicle,” a missile payload containing several warheads, each of which can be sent to a different target. The report said the test had involved part of a MIRV system, not a full-fledged multiple-warhead missile.

Since North Korea’s leader, Kim Jong-un, hosted Mr. Putin for talks last week, officials and analysts have expressed concern that their deepening ties would encourage Mr. Kim to embark on an ambitious upgrade to his nuclear arsenal.

MIRV capability would be a drastic increase in the threat the North poses to the United States and its allies, because a high-speed ballistic missile splitting into several nuclear warheads, as well as decoys, is harder for missile defense systems to intercept. But experts believe the North is far from mastering the technology.

Col. Lee Sung-jun, a spokesman for the South Korean military, said on Thursday that there was “deception and exaggeration” in the North’s announcement. He did not elaborate, but he said photos of the test carried by state media may have been altered. South Korean officials dismissed the Wednesday test as a failure soon after it occurred, saying a missile had exploded over waters east of North Korea after flying 150 miles. They said the test appeared to have involved a hypersonic ballistic missile.

Under multiple United Nations Security Council resolutions, North Korea is forbidden to develop or test nuclear or ballistic missile technologies. But Mr. Kim has doubled down on expanding those capabilities since 2019, when his direct diplomacy with then-President Donald J. Trump collapsed.