The experts from the International Atomic Energy Agency (IAEA) were to meet in Tehran on Thursday with Iranian officials to discuss modalities for the inspections to be carried out ahead of an October 31 deadline given to Iran to come clean on its nuclear program.
"We do expect that we will make a lot of progress with the full cooperation of Iran and that's why I'm going," said Pierre Goldschmidt, an IAEA deputy director general who oversees the department of safeguards.
Goldschmidt said he expected Iranian officials to "have a very important message for us" on extending cooperation "and that we can progress very rapidly because as you know we have only a few weeks to make progress."
IAEA chief Mohamed ElBaradei had said Tuesday at IAEA headquarters in Vienna: "I think this is a decisive moment in our work in verifying Iran's nuclear programme."
ElBaradei said the IAEA was now waiting for "full disclosure by Iran" to address claims that Tehran is seeking to develop nuclear weapons, a charge Tehran denies.
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.
In Tehran, the government declared Wednesday that it was determined to avoid an all-out row with the IAEA as it wanted to keep the issue out of the Security Council.
"This question must not be sent to the Security Council. This must be prevented," Foreign Minister Kamal Kharazi was quoted as saying by the state news agency IRNA.
Government spokesman Abdollah Ramazanzadeh echoed the view, telling reporters that "we do not want this question to go to the Security Council and will take all measures to reach an accord with the agency."
But while promising "greater cooperation" with UN inspectors, Kharazi remained circumspect on demands the country sign an additional protocol to the nuclear Non-Proliferation Treaty (NPT) allowing unlimited and surprise visits to suspect sites.
Iran has been asked to cease uranium enrichment, following the discovery during previous visits by IAEA inspectors of traces of highly enriched uranium that could be weapons-grade.
Iranian officials claim the uranium particles into the country on imported equipment.
Goldschmidt said he would first seek to resolve "technical matters" with Iranian officials and then IAEA "experts and inspectors" would be following him into Tehran "very rapidly and we'll be there for the rest of the month."
The United States has branded Iran part of an "axis of evil," and alleges that Tehran has already tested centrifuges used to make highly enriched uranium.
Iran said Monday that it would not accept any restrictions on its bid to generate nuclear power, reserving the right to enrich uranium for fuel.