"The first mission is leaving on Sunday," Melissa Fleming said, to be headed by Pierre Goldschmidt, a Belgian who is an International Atomic Energy Agency (IAEA) deputy director and responsible for non-proliferation safeguards.
Fleming said the IAEA had given Iran "a work plan covering the whole month of October," right up until the deadline.
The IAEA imposed the deadline on Iran on September 12 and also urged it to suspend enriching uranium which the United States claims could be used to make nuclear bombs.
But Iran announced Monday that it had launched a trial run at a uranium enrichment factory in Natanz, at the centre of Western concerns over its nuclear programme.
Fleming said: "We routinely go to Natanz to do safeguards inspections but there will be further inspections."
She said five to six IAEA inspectors, who are to arrive in Iran next week, would be going to Natanz.
The IAEA wants to check into components contaminated with highly enriched uranium.
Iran claims the contamination is not from its enrichment activities but from work done in countries from which the second-hand components were imported.
The IAEA has asked Iran to give it "a list of equipment and components" it has imported.
Fleming said the agency also needed information from the countries from which they originated.
Iran had said it was fully committed to cooperating with the agency and honoring the nuclear Non-Proliferation Treaty (NPT), despite its objections to being handed a deadline.
But Ali Akbar Salehi, Iran's ambassador to the IAEA, said in a newspaper report in Tehran Monday that Iran had so far "cooperated beyond the (current nuclear anti-proliferation) accords and allowed the taking of (environmental) samples and inspections of non-nuclear sites."
Salehi said that in the future Iran would go no further than its commitments under the NPT, "while completing what has been embarked upon."
He said Iran would however continue talks with the IAEA on signing an additional protocol to allow IAEA inspectors to make surprise visits to suspect sites.
Fleming said the IAEA was asking Iran to let it act as if the protocol was already in place, "for the sake of transparency and accomplishing requirements to meet this end-of-October deadline."
Fleming said the the IAEA had "received nothing officially" from Tehran in the line of Salehi's comments.
"This plan has been submitted to Iran so they are fully aware of what is required from their side and also from the work that the IAEA needs to do over this next period until October 31," Fleming said.
She said the IAEA investigation will include "environmental sampling and access and receiving information from their side."
A key issue with the IAEA has been Iran's delaying on its requests to do soil and other sampling at sites where nuclear activity is suspected.
Iran denies it is seeking to acquire nuclear weapons, but its failure to comply with the IAEA deadline could lead to Tehran being declared in non-compliance with the nuclear Non-Proliferation Treaty (NPT), with the matter being passed to the UN Security Council.