The absence of Russia and China from the two days of talks in Vancouver, which began on Monday, shows the holes in Washington's bid to form a unified global front against North Korea's nuclear threat.
"The most important relevant parties of the Korean peninsula issue haven't taken part in the meeting so I don't think the meeting is legal or representative," Chinese foreign ministry spokesman Lu Kang told a regular briefing.
Lu denounced the "Cold War mentality" of "relevant parties" -- without naming nations.
China, which is North Korea's main economic and diplomatic ally, has criticised the Vancouver talks and called for sanctions discussions to remain within the United Nations framework.
While Russia and China have backed UN sanctions against North Korea, they have pushed for the United States to halt military drills in the region in return for Pyongyang suspending weapons tests.
Steve Goldstein, a senior US State Department official, told reporters last week that China and Russia were not invited to the meeting but would be briefed about it "right when it is over".
The US, which at the summit will review with its allies the effectiveness of current sanctions against the hermit kingdom and explore adding more, remains sceptical that leader Kim Jong-Un is ready to negotiate away his weapons programme.
The so-called Vancouver Group is formed by 20 countries that fought in the 1950-53 Korean War. They include Australia, Britain, France, India, Japan, the Philippines and South Korea. Military officials will also be present at the meeting.