However, the deputy head of the Iranian Atomic Energy Organization, Mohammad Saaidi, stressed that production of nuclear fuel remains suspended.
The move puts Iran at odds with some Western nations, who fear Iran may be using the research in an effort to develop a nuclear weapon.
In Brussels, the European Union said the Iranian action violates an agreement with the EU to hold back from sensitive nuclear activities. Cristina Gallach, a spokeswoman for EU policy chief Javier Solana, called it "very much a step in the wrong direction."
Iran says it is pursuing a peaceful nuclear program.
Moscow said its offer to enrich uranium jointly with Iran at a site in Russia remains open and that it is still waiting for an answer from Tehran.