The State Department said Iran's signing of an additional protocol to the nuclear Non-Proliferation Treaty (NPT) that allows the snap visits was only a "first step" in addressing US accusations that Tehran is secretly developing atomic weapons.

"It is welcome that Iran has made this commitment, but what is important remember is that it is only a first step," spokesman Richard Boucher said after Iran signed the protocol earlier Thursday at the headquarters of the International Atomic Energy Agency (IAEA) in Vienna.

"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 programs that they have agreed to," he said.

The United States accuses Iran of using what it claims to be a civilian atomic energy program to hide a covert nuclear weapons program.

Iran vehemently denied the charges but had long resisted signing the additional protocol for the surprise inspections, claiming it would be violation of sovereignty and open the door to espionage.

Tehran made an about-face under intense diplomatic pressure in October when the foreign ministers EU countries Britain, France and Germany visited the country and agreed to sign the additional protocol, hand over full details of its activities and suspend uranium enrichment.

In November, the IAEA adopted a resolution condemning Iran for 18 years of covert nuclear activity after its chief issued a report that detailed violations of the NPT but found no evidence of a nuclear weapons program.

Boucher, echoing remarks made by US ambassador to the IAEA Kenneth Brill in Vienna, said the IAEA had to ensure that its inspections were robust given its finding that Iran had deceived it in the past.

"They need to ensure that there is regular verification of the protocol's implementation because Iran does have a history of deception in this area," he said.

But, Boucher stressed, ultimately only full compliance with IAEA demands would ease its and other international suspicion of Iran's nuclear program.

"Fundamentally, for the international community to have full confidence in Iran's nuclear program, they are going to need to abandon enrichment and reprocessing and going to need to cooperate fully with the (IAEA)," he said.

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