Iran signed an additonal protocol to the nuclear Non-Proliferation Treaty (NPT) at the Vienna headquarters of the UN nuclear watchdog agency, the International Atomic Energy Agency (IAEA).
"I welcome the signature today by Iran of the IAEA additional protocol as an important step in building international confidence about the peaceful nature of Irans nuclear programme," EU foreign policy envoy Javier Solana said.
The move capped months of intense diplomacy to pressure Tehran into placing its nuclear facilities under greater international supervision and prove that it is not seeking to develop atomic weapons.
"Full continuous implementation of this protocol as well as compliance with IAEA board of governors resolution are crucial in this respect," Solana said in a statement, referring to the board's recent condemnation of Iran's covert nuclear activities over two decades.
European External Affairs Commissioner Chris Patten also described Iran's decision as good news.
"It will help in establishing the international community's confidence in Iran's assurance about the peaceful nature of its nuclear programme and its commitment to non-proliferation," he said.
"Ratification and implementation pending the entry into force of the AP (additional protocol) is now essential to provide full transparency on the Iranian nuclear programme."