Bolat Nurgaliyev said the question of enlargement "is not on the agenda yet."
However, Nurgaliyev indicated that the organization was open to cooperation with non-members, to which end the status of "SCO dialogue partner" is to be introduced at a SCO heads of state meeting in Dushanbe on August 28.
"The new status will be granted to states or international organizations that are interested in cooperation with the SCO in some area or other," he said.
Iran and Pakistan previously announced their desire to become permanent members of the organization, but a SCO foreign ministers meeting in Tajikistan on July 25 did not consider their request.
Iranian Security Council chief Saeed Jalili said earlier on Wednesday that the Islamic Republic "has great potential for ensuring peace and progress in the world."
"This potential is a good opportunity for the Shanghai Cooperation Organization," he told Tajik Foreign Minister Khamrokhon Zarifi, on a visit to Tehran.
The SCO imposed a moratorium on new admissions two years ago.
Iran and Pakistan, observer states at the SCO since 2005, have sought full membership in the post-Soviet regional bloc comprising Russia, China, Kazakhstan, Tajikistan, Kyrgyzstan, and Uzbekistan. The SCO is widely seen as a counterweight to NATO's influence in Eurasia.
The group primarily addresses security issues, but has recently moved to embrace various economic and energy projects.
Russia and China have been cautious over admitting Iran, embroiled in a long-running dispute with the West and Israel over its nuclear program and alleged support for radical groups in Lebanon and other countries.
Both China and Russia have major commercial interests in Iran. China wants Iranian oil and gas, and to sell weapons and other goods to the country, while Moscow hopes to sell more weapons and nuclear energy technology to Tehran.
The Kremlin also needs Iran's endorsement for a multinational arrangement to exploit the Caspian Sea's energy resources.
The other observers in the group are India and Mongolia.
