China supports Iran's membership of the Shanghai Cooperation Organization (SCO) security bloc, jointly led by China and Russia, and the subject will be discussed at the group's summit this week, a senior diplomat said on Monday.

The SCO refused to initiate Iran's accession last year despite a request from Russia which backs Tehran's bid.

Assistant Chinese Foreign Minister Li Huilai said Iran is an observer at the SCO and has for a long time "proactively participated" in its activities and has made positive contributions to the SCO's development, according to channelnewsasia.com.

"China highly appraises this. China welcomes and supports Iran's wish to become a formal member of the SCO," he told reporters, ahead of the summit in Kazakh capital Astana which President Xi Jinping will attend.

"I think that at this meeting all sides will continue to conscientiously study the issue of Iran becoming a member on the basis of the SCO's relevant rules and consensus through consultations."

China has close economic and diplomatic ties with Tehran, and was also instrumental in pushing through a landmark nuclear deal with Iran in 2015.

China said in November it was willing to consider any application from NATO-member Turkey to join the SCO, after Turkish President Tayyip Erdogan said his country could join.

China, Russia, Kazakhstan, Kyrgyzstan, Tajikistan and Uzbekistan formed the SCO in 2001 to fight threats posed by radical Islam and drug trafficking from neighbouring Afghanistan.