Iran applied in 2008 to join the Shanghai Cooperation Organization (SCO), a security and defense bloc led by Beijing and Moscow and seen as a counterweight to NATO.
The SCO has not officially given a verdict on the Iranian application but Russia's ITAR-TASS news agency quoted the source, speaking during a meeting of SCO foreign ministers in Uzbekistan, as saying that sanctions were an obstacle.
"A country which wants to become a member of the SCO cannot be subject to UN Security Council sanctions," the Russian source said.
No SCO candidate member apart from Iran has UN sanctions applied against it.
The SCO secretary-general, Muratbek Imanaliyev, had said in February that the bloc was assessing Iran and Pakistan's applications for membership but did not express a view.
At present the SCO membership comprises China, Kazakhstan, Kyrgyzstan, Russia, Tajikistan and Uzbekistan. The bloc's activities have focused on military cooperation, intelligence-sharing and the fight against terrorism and drugs.
Iran, Pakistan, Mongolia and India are observers, while Belarus and Sri Lanka have the lesser status of dialogue partners.
The organization has no formal process for admitting new members and ITAR-TASS reported that the SCO foreign ministers had agreed rules on this to present for approval to the next SCO heads of government summit on June 10-11.
Moscow and Beijing have so far been reluctant to admit Tehran because of Western concerns about the country's nuclear program, analysts say. Moscow would like to see its close military and strategic ally India become a full member of the SCO but New Delhi has shown little interest so far.
