"I think this is a decisive moment in our work in verifying Iran's nuclear programme," Mohamed ElBaradei, head of the International Atomic Energy Agency (IAEA) told reporters.
The inspectors are scheduled to leave Vienna on Wednesday and talks are due to begin Thursday in Tehran on verifying nuclear sites.
The IAEA has set October 31 as a deadline for Iran to prove that it is not secretly developing nuclear weapons -- a charge Tehran denies.
ElBaradei said the IAEA was now waiting for "full disclosure by Iran" to address claims that Tehran is seeking to develop nuclear weapons.
"We need full transparency. We need full disclosure to resolve the outstanding issues," said ElBaradei.
The United States said Monday it would press for Iran's nuclear program to be referred to the UN Security Council for possible sanctions if Tehran does not meet the deadline.
"The next few weeks are crucial to gauge the level of cooperation and transparency by Iran," ElBaradei said.
Inspections of the nuclear sites are to take place throughout October right up to the deadline at the end of the month.
ElBaradei said the "most important outstanding issue" was Iran's program to enrich uranium, with the United States claiming Tehran could produce highly enriched uranium for atomic weapons.
Iran insists its program is purely for civilian energy purposes and has bristled at IAEA demands for greater access to its sites.
Iran's representative to the IAEA has said traces of highly enriched uranium found by IAEA inspectors in August at the Kalaye Electric Company near Tehran and earlier this year at the Natanz fuel production facility in southern Iran came into the country on imported equipment.
The IAEA director said the agency was meanhwile getting information from "different sources" about supplies to Iran, but did not provide details.
ElBaradei rejected US demands for full disclosure about its nuclear program and a new mecanism for surprise inspections by the October 31 deadline, saying that the IAEA was prepared to abide by a longer timetable if Iran cooperated.
"I am not going to scuttle the process because of a particular date nor am I going to jump to conclusions," he said.
The US State Department said in Washington that Iran must answer questions about its program and accept a new IAEA protocol which would allow surprise inspections of its nuclear sites by the agency's October 31 deadline or face possible UN action.
ElBaradei said the IAEA had not yet "started a discussion" with Iran on a new protocol but that this was not his "number one priority" as the protocol was more important for future monitoring.
"My number one priority (now) is to resolve past outstanding issues," ElBaradei said.
The United States has branded Iran part of an "axis of evil," and alleges that Tehran has used the Kalaye site to test centrifuges used to make highly enriched uranium.
Iran said earlier Monday that it would not accept any restrictions on its bid to generate nuclear power, reserving the right to enrich uranium for fuel.