The United States and South Korea pledged to make Russia and North Korea "pay the price" for military cooperation, as they urged Moscow to act responsibly as a permanent member of the U.N. Security Council, according to the South Korean deputy foreign minister.

Earlier this week, the leaders of Moscow and Pyongyang had an unusual summit meeting.

The fourth session of the Extended Deterrence Strategy and Consultation Group (EDSCG), which brings together deputy defense officials and diplomats from allied countries, convened at Seoul’s Foreign Ministry amidst worries that North Korea and Russia might have deliberated on a potential arms transaction during their recent summit, Seoul-based Yonhap News reported.

The gathering was chaired by South Korea’s Vice Foreign Minister Chang Ho-jin and Vice Defense Minister Shin Beom-chul, whereas the U.S. delegation was led by Undersecretary of State for Arms Control and International Security Bonnie Jenkins and Deputy Undersecretary of Defense for Policy Sasha Baker.

Seoul and Washington cautioned that military cooperation between Pyongyang and Moscow is in breach of U.N. Security Council resolutions and will have consequences.

"The recent reports about the potential sale of arms between North Korea and Russia are concerning and any such transfer of arms would violate U.N. Security Council resolutions," said Bonnie Jenkins during a news conference after the meeting.

Jenkins reiterated that the U.S. maintains its stance that any nuclear attack by North Korea on Washington or its allies, including South Korea, would be deemed "unacceptable and result at the end of that regime."

In a much-watched meeting, Russian President Vladimir Putin and North Korean leader Kim Jung-un met Wednesday in Amur in Russia’s Far East.

Before the meeting, Kim said they wanted to enhance their relationship with Russia further, adding that he always supported Putin and the Russian government’s decisions.

Putin also described his meeting with Kim as "productive."