"We have agreed to do it, so from our point of view it is definite. Now the issue is in its preliminary phase and the cabinet is finalising the signing of the text," foreign ministry spokesman Hamid Reza Asefi told reporters.
"After the signing there is another process and it will be presented to the Majlis (parliament). We are sticking to our obligations and we will sign it," he added.
Signing the additional protocol would subject Iran to surprise inspections of its nuclear facilities, which Washington claims are used by Tehran as cover for an atomic weapons programme. The allegations are fiercely denied by the Islamic republic.
The International Atomic Energy Agency (IAEA) last month condemned Iran for 18 years of covert nuclear activities but stopped short of bowing to US demands of hauling Tehran in front of the UN Security Council for possible sanctions.
Iran escaped a tougher line by the UN watchdog after it agreed to fully disclose its activities to the IAEA, sign up to tougher inspections and suspend uranium enrichment.
On Thursday, IAEA director general Mohamed ElBaradei said he expected Iran would sign the additional protocol in the "next couple of weeks" -- although Iran has been accused of stalling on the issue.
Asefi also reiterated that Iran's suspension of its controversial work on the nuclear fuel cycle -- seen as putting it on the path to eventually developing nuclear weapons -- was only temporary.
"The issue of stopping the enrichment of uranium is not the question. The question is of a voluntary and temporary suspension," Asefi said.
"The question of stopping is not up for discussion. Iran will not give up its right to use peaceful nuclear energy," he added.
And the spokesman also brushed off suspicions that Iran had sourced much of its nuclear equipment -- especially centrifuges -- from neighbouring Pakistan.
"I deny such reports we had any cooperation with Pakistan on the nuclear issue," he said.