The United States on Thursday expressed deep concern over Russian President Vladimir Putin's remarks that he does not rule out supplying the DPRK with weapons.

"It is incredibly concerning" that the Russian president "does not rule out" shipments to Pyongyang, State Department spokesman Matthew Miller said.

"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."

Earlier in the day at a press conference during his visit to Vietnam, Putin asserted Russia's readiness to supply arms to other countries in response to Western arms transfers to Ukraine, citing Russia's agreements, including those with DPRK, as grounds for potential actions.

"Those who are supplying these weapons believe that they are not at war with us, well, I have said, including in Pyongyang, that we reserve the right to supply weapons to other regions, bearing in mind our agreements with the Democratic People's Republic of Korea, I don’t rule out this as well, where will these [weapons] go," Putin told reporters.

After the DPRK and Russia agreed to defend one another in the event of war, South Korea announced on Thursday that it would think about transferring armaments to Ukraine. This is a significant shift in policy.

The agreement was denounced by the office of South Korean President Yoon Suk Yeol, who also warned that Seoul's relations with Moscow will suffer as a result. The statement described the agreement as a breach of resolutions passed by the U.N. Security Council and a threat to his country's security.

Chang Ho-jin, Yoon's national security advisor, stated that Seoul will reevaluate the question of arming Ukraine to aid in fending off Russia's full-scale invasion.

On Thursday, Russian President Vladimir Putin issued a warning to South Korea, saying that supplying Ukraine with deadly weapons would be a "big mistake."

“If South Korea supplies weapons to Ukraine, it will not like the answer. I hope they won’t do it, it would be a big mistake,” Putin said at a press conference in Vietnam, where he is conducting a two-day state visit aimed at deepening strategic cooperation.

Read more: Russia, DPRK committed to mutual aid in case of armed aggression

No reasons to change US nuclear posture

In light of Putin's remarks, Pentagon Press Secretary Air Force Maj. Gen. Pat Ryder stated that Washington has seen no compelling reasons that would require changes to the US' nuclear posture.

"We've seen nothing at this point that would require us to change our own strategic forces posture," Ryder said in a press briefing when asked about Putin's remarks.

The Russian president announced earlier that Russia is contemplating revisions to its nuclear doctrine in response to recent developments in Western strategic thinking that could lower the threshold for using nuclear weapons.

Putin expressed concerns over emerging technologies, including ultra-low power nuclear devices, being discussed in Western expert circles, stating, "Now we are also thinking about what and how could be changed in the nuclear doctrine, in the strategy. And it is connected with this. It is connected with the fact that there are new — in any case, we know that the probable enemy is working on it — new elements related to lowering the threshold for the use of nuclear weapons."

He dismissed the necessity of a preemptive nuclear strike clause in Russia's doctrine as he expressed confidence in Russia's capability to respond effectively to any aggression with a retaliatory strike.

Read more: US 15 years behind China in nuclear power development: Report