"It demonstrates how dependent President Putin, and Moscow, now is on authoritarian countries across the world," Secretary General Jens Stoltenberg told reporters in Washington following a speech at the Wilson Center.

"Their closest friends and the biggest supporters of the Russian war effort -- war of aggression -- (are) North Korea, Iran and China," Stoltenberg said.

Asked what actions NATO could take, Stoltenberg said that "there are many sanctions already on North Korea. The problem is that Russia is now violating those sanctions."

Using a figure given previously by South Korean intelligence, Stoltenberg said that North Korea has already delivered one million shells to Russia.

"That flow of weapons continues," Stoltenberg said. "We see them loading the railroad cars and then crossing the border between North Korea and Russia."

Russia is a veto-wielding member of the UN Security Council, which has imposed a slew of sanctions on North Korea over its nuclear and missile programs.