In the late 1950s, a number of communist-ruled non-member countries the People's Republic of China, North Korea, Mongolia, Vietnam, and Yugoslavia were invited to participate as observers in Comecon sessions. Although Mongolia and Vietnam later gained full membership, China stopped attending Comecon sessions after 1961. Yugoslavia negotiated a form of associate status in the organization, specified in its 1964 agreement with Comecon. Collectively, the members of the Comecon did not display the necessary prerequisites for economic integration: their level of industrialization was low and uneven, with a single dominant member (the Soviet Union) producing 70% of the community national product.