Iranian nuclear negotiator Saeed Jalili said uranium enrichment to the 20 percent purity level was needed for medical research. Domestic enrichment began as a response to opposition to supplying the material.
"The nuclear equation has changed and the enemies want Iran to only halt 20-percent uranium enrichment," he was quoted by state-funded broadcaster Press TV as saying.
Iran said recently that nuclear researchers may consider nuclear fuel enrichment levels of uranium beyond 20 percent, a level considered a breakthrough on the path toward a nuclear weapon.
Jalili took part in multiparty nuclear negotiations in Kazakhstan this year. Those talks yielded few breakthroughs and the International Atomic Energy Agency said recently there were lingering concerns about Iran's peaceful intent.
Jalili is one of eight candidates approved by the 12-member Guardian Council and will be on the presidential ballot June 14.
Iranian President Mahmoud Ahmadinejad is ineligible to compete because of term limits.
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