The United States expressed deep concern Thursday over Russian President Vladimir Putin's threat to supply North Korea with weapons, warning such a move would "destabilize" the Korean peninsula.

Putin, during a rare visit to Pyongyang, signed a mutual defense pact on Wednesday with North Korean leader Kim Jong Un, who pledged his country's "full support" for Russia's invasion of Ukraine.

Speaking on Thursday in Vietnam, Putin said Moscow would not rule out sending weapons to Pyongyang, calling it repercussions for the West supplying Ukraine.

The threat "is incredibly concerning," State Department spokesman Matthew Miller told reporters.

"It would destabilize the Korean peninsula, potentially, depending on the type of weapons, and might violate UN Security Council resolutions that Russia itself has supported," Miller said.

Washington and its allies have previously accused North Korea of supplying Russia with missiles and artillery that it has used to attack Ukraine.

Putin warned Seoul on Thursday not to supply Ukraine with weapons, after South Korea said it was reconsidering its current ban.

Seoul has a longstanding policy that bars it from selling weapons into active conflict zones, which it has stuck to despite calls from Washington and Kyiv to reconsider.

Miller said such a decision was "for every country to make in terms of whether they're going to supply weapons to Ukraine."

"We welcome any support for Ukraine in its fight against Russian aggression," he added.