The State Department said Iran's accession to the additional International Atomic Energy Agency (IAEA) protocol would be only one step in dealing with suspicions that Tehran is secretly trying to develop nuclear weapons.
"We'd certainly welcome Iran signing of the additional protocol, but the signature is only one step toward resolving the remaining open questions about Iran's nuclear program," spokesman Richard Boucher said.
It is only one step "toward increasing international confidence that Iran's nuclear program will be limited to peaceful activities and that they will truly suspend all uranium enrichment-related and reprocessing activities," he said.
To fully convince Washington and the rest of the world, Boucher said Tehran would have to completely implement the provisions of the protocol and provide the IAEA will all the access it needs for the inspections.
Earlier Wednesday, the IAEA, which last month found Iran to have failed to meet all of its commitments to the nuclear Non-Proliferation Treaty (NPT), said Iran would sign the additional protocol on Thursday.
The United States accuses Iran of using its civil atomic energy program to mask nuclear weapons development but the IAEA has said there is no clear evidence to support that charge.
Iran vehemently denies the US allegations but had long resisted signing the additional protocol.
Tehran argued that inspectors could violate its national sovereignty and probe sites that are crucial to the defense of the country and demanded guarantees this would not happen.
But Iran finally gave in to IAEA demands after the agency threatened to refer its concerns to the UN Security Council, which would have left Iran vulnerable to sanctions.
That U-turn came in October during an unprecedented visit to Tehran by the foreign ministers of the European Union's big three -- Britain, France and Germany after which Iran agreed to sign the additional protocol, hand over full details of its activities and suspend uranium enrichment.