Although China provides the bulk of North Korea's oil, Russia exports 40,000 tonnes of oil per quarter to the isolated state, Mr Putin said on Tuesday.

But he called this “nothing” compared to its overall output and denied that the North Korean workers in Russia, who provide cash to the regime, were a problem.

He added that the North Koreans “will eat grass but they won't give up (the nuclear) programme if they don't feel safe”.

Mr Putin and Xi Jinping, the Chinese president, have instead pushed a “freeze for freeze” plan that would see the United States and South Korea stop large military exercises in exchange for North Korea ceasing weapons tests.

Jerry Brown, the governor of California, also spoke at the forum, calling on Russia, China, Japan and the United States to fight climate change.

Mr Brown promised to help Russian students with visas after Washington closed Russia's San Francisco consulate this weekend.