Iran’s outgoing representative to the Vienna-based United Nations nuclear watchdog, Ali-Akbar Salehi, in a ceremony attended by IAEA’s Egyptian Chief Mohammad el-Barade’i, signed the document.

Tehran strongly deny that it is building an atomic arsenal, claiming that all its ongoing projects are aimed at "pacific" and civilian use, mostly producing electricity, a claim rejected by experts on the basis of having the largest natural gas reserves after Russia, the country does not need expensive and complicated atomic technology, mostly the ageing and not much reliable one provided by Russia.

"This is the opportunity for Iran to break a vicious circle that has been going on for 20 years", Mr. el-Barade’i said, referring to some of Iran’s nuclear activities, including enriching uranium that had been hidden from inspections.

Kenneth Brill, the U.S. representative at the IAEA called the accord "a useful step in the right direction", but said only aggressive inspections would erase doubts sown by Iran's "nearly two decades of deception".

"Iran needs to bring this into force, needs to ratify this additional protocol that is now signed and above all it needs to implement the programmes that they have agreed to", he said.

"It is a landmark event, I hope that now my country will not be exposed anymore to unfair and politically motivated accusations", Salehi told reporters wile bidding farewell, being replaced by the Iranian Ambassador to Austria.

"It is welcome that Iran has made this commitment, but what is important remember is that it is only a first step", State Department spokesman Richard Boucher said in Washington.

The Bush administration will keep an eye on what Iran actually does about its nuclear program now that it has signed a key accord to open its facilities to international inspections, the American news agency The Associated Press quoted un-named senior U.S. officials as having on Thursday. "The real question, they said, was whether Iran would honour the commitments in the accord.

Head of the 35-members Board of the Governors of the IAEA, Antonio Nunez Garcia-Sauco said that the Board expects Iran to immediately implement the Protocol after it’s signing; the official Iranian news agency IRNA reported signed the protocol.

He welcomed the "confidence-building" measure taken by Iran to sign the NPT additional protocol, hence fulfilling one of the IAEA’s demands as mentioned in its 26 November declaration, adding that full transparency of nuclear activities in the past and present as well as suspension of the uranium enrichment program had been among other IAEA’s expectations.

He added that the Board is now busy studying a report on Iran’s nuclear activities and that the country has suspended its uranium enrichment activities, according to IRNA.

He expressed hope that Iran will fully abide by the NPT protocol so as to please both sides.

Asked to comment on transfer of nuclear technology to Iran, Garcia said technology transfer is a very long process, having different phases, with the NPT Additional Protocol making up the first stage.

"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 Iran's nuclear programme", EU’s Foreign and Security Affairs Minister Javier Solana said.

The EU announced that Solana would visit Tehran in the beginning of next year in a trip aimed at boosting economic and political cooperation.

Iran agreed to sign the Protocol two months after foreign ministers of Britain, France and Germany, during a snap visit to Tehran on 20 November, warned Iran that if it refuses not to comply with the IAEA’s demands, the matter would be sent to the United Nations Security Council to decide drastic economic sanctions against the Islamic Republic of Iran, a regime already isolated on the international scene.

Hojjatoleslam Hasan Rohani, the powerful Secretary to Iran’s Supreme Council for National Security, a cleric close to the regime’s leader Ayatollah Ali Khameneh'i then announced that Iran would sign the Protocol and suspend uranium enriching programmes provided its national security and sensibilities are not harmed and that IAEA would transfer to Iran nuclear technologies for peaceful uses.