The US said it was concerned about Russian President Vladimir Putin's threat to supply North Korea with weapons, warning it would "destabilize" the Korean peninsula.
Depending on the type of weapons provided, such a move may violate UN Security Council resolutions which Russia itself supported, US State Department spokesman Matthew Miller said on Thursday.
The US would continue to work with its allies in the region, including South Korea and Japan, to "respond to the threat posed by North Korea," he said.
During a state visit to Pyongyang on Wednesday, Putin signed a partnership agreement with North Korean strongman Kim Jong Un, that includes a vow of mutual aid if either country is attacked.
Speaking in Vietnam on Thursday, Putin threatened to deliver high-precision weapons to North Korea as a response to Western arms deliveries to Ukraine.
The South Korean government expressed concern over the Russia-North Korea strategic partnership.
Seoul said the security commitments violated UN sanctions imposed on Pyongyang. South Korea also suggested it could reconsider its long-standing policy of not supplying weapons to Kiev.
Putin said South Korea would be making a "big mistake" if it supplies arms to Ukraine.
"If that happens, then we will make corresponding decisions that will not please the current leadership of South Korea," Putin said.
Putin called Seoul's concerns unfounded.
He said South Korea had nothing to fear as the pact would only apply if either Russia or North Korea was attacked by a third country.
"As far as I know, [South Korea] is not planning any aggression" against North Korea, he said. This means that "there is no need to fear our cooperation."
Putin also said that he would not deploy any North Korean soldiers in Ukraine.
North Korea, ruled by dictator Kim Jong Un, is subject to far-reaching UN sanctions and import bans due to its nuclear weapons programme, including the trade in weapons and the transfer of military technologies to the country.
