Lu is in Tehran for talks with his Iranian counterpart on the future of Iran's nuclear program. The letter was delivered to President Mahmud Ahmadinejad.
His visit coincides with that of Russian Federal Atomic Energy Agency chief Sergei Kiriyenko, who says Russia's offer to enrich Iranian uranium on Russian territory is still on the table.
China supports the Russian proposal.
Iran's Nuclear Program
THE COMPLETE PICTURE: RFE/RL's complete coverage of controversy surrounding Iran's nuclear program.
An annotated timeline
of Iran's nuclear program.