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Iran on Thursday signed a memorandum on joining the Russian state-owned news agency TASS reported.

Founded in 2001, the Shanghai Cooperation Organisation is a security bloc comprising Russia, China, India, Pakistan and a number of post-Soviet states.

Iran was previously an observer member.Ìý

¸é³Ü²õ²õ¾±²¹'²õÌýand China'sÌýwill gather with other Asian leaders in the ancient Silk Road city of SamarkandÌýfor aÌýregional SCO summitÌýtouted as a challenge to Western global influence.

They will be joined by the leaders of India, Pakistan, Turkey, Iran and several other countries.Ìý

The main summit day will be Friday, but it is a meeting of the Russian and Chinese leaders on Thursday that will be the most closely watched.

Iran, one of four SCO observer states, applied for full membership in 2008 but its bid was slowed by UN and US imposed over its nuclear programme.

Several SCO members did not want a country under international sanctions in their ranks.

However, at a conference in Dushanbe in September last year, members of the bloc endorsed Iran's future membership.

Tehran is current engaged in high-stakes talks with Western powers over revitalising a which if finalised would put an end to international sanctions in exchange for curbs on Iran's uranium enrichment.Ìý