The China-led Shanghai Cooperation Organization has refused to enlist Iran, despite a request from Russia.
While begun in China, the Asian security bloc has served as a platform for both Moscow and Beijing to project influence in Central and South Asia. But unlike Russia, China is showing some reluctance to give it a strong anti-Western bias.
Iran has long sought to join the group, and Russia has argued that with Western sanctions against Tehran lifted, it should finally become a member.
"The Russian position is clear in its support of initiating the process for Iran's admission without delays," Bakhtiyor Khakimov, Russian President Vladimir Putin's envoy to the organization, told reporters as bloc members met in Uzbekistan on June 23.
Khakimov said there were no objections to the idea "in principle," but there were "technical nuances" related to the timing that caused Iran's bid to fail.
Chinese diplomats at the meeting did not comment, but Chinese Foreign Minister Wang Yi has said Beijing wanted to focus on the ongoing accession of India and Pakistan before moving on.
Based on reporting by Reuters and Interfax