The promise to allow access to the Lavizan site was made this week in a letter to the International Atomic Energy Agency (IAEA) in Vienna, to the deputy director for safeguards, Ollie Heinonen, who left for Iran on Tuesday.
IAEA officials refused to comment on whether Mr Heinonen, who is expected to return from Iran this weekend, has actually visited Lavizan, which the United States claims is a site used for work related to atomic weapons.
Iran tried to acquire equipment that could have been used for uranium enrichment at the Lavizan site, the IAEA said in a 2004 report.
Iran has removed buildings and topsoil from Lavizan but IAEA inspectors, who have already visited the site, still want to take environmental samples to check for traces of uranium and want to investigate dual-use machines used at Lavizan, which they say could be either for civilian or weapons purposes.
IAEA meeting
Mr Heinonen's trip gives Iran a last chance to comply with international inspections, ahead of an emergency IAEA meeting on the Iran’s nuclear program in Vienna next Thursday.
Britain, Germany and France (the EU-3) called for the meeting after Iran resumed uranium enrichment work on January 10. Enriched uranium can be used either as fuel for atomic reactors or the raw material for nuclear weapons.
The EU-3 and the US want to refer Iran to the UN Security Council, which has enforcement powers such as sanctions, to pressure Tehran to cease nuclear fuel work and to comply with a now three-year-old IAEA investigation into an Iranian atomic program, which the US charges is a cover for secret weapons work.
Diplomats have told news agency AFP that Iran may have received in 1997 three P-2 centrifuges, which are capable of enriching uranium, from the black-market network of disgraced Pakistani nuclear scientist Abdul Qadeer Khan.
Iran denies having received imports of such centrifuges.
Mr Heinonen is due to file a progress report ahead of the February 2 meeting, with a more detailed account coming in March.
Diplomats said Iran promised to provide responses to key questions the IAEA says must be answered.
The questions concern offers of nuclear technology and parts Iran received in 1987 and 1994 from the Khan network.
The IAEA has also asked Iran to supply a document it showed the agency on making hemispheres of uranium metal, objects whose only use is as the core of atom bombs.
The final question concerns Iran's alleged efforts to work directly on making nuclear weapons, such as US intelligence that Tehran worked on putting payloads on missiles in a form only suitable for nuclear bombs.
Russian plan mulled
Meanwhile a day after President George W Bush gave his support to a Russian proposal to end the Iran nuclear crisis, the US administration said it does not accept every detail.
The US State Department cast new doubt on Russia's offer to manufacture nuclear fuel for Iran and collect the waste so that it cannot be used to develop a nuclear bomb.
"The United States has said that we find the Russian proposal to be interesting and it might be a good way to proceed with negotiations. We've never said that we accept every detail in that proposal," said Nicholas Burns, the assistant secretary of state for political affairs.
Mr Bush gave high profile backing to the Russian plan at a White House press conference on Thursday.
"We've said that we believe that this kind of proposal has promise, but we've never blessed every article of the proposals. I think if you ask the Russian government, they'd tell you the same thing,” Mr Burns said on Friday.
US Secretary of State Condoleezza Rice will meet in London on Monday with counterparts from Britain, China, France, Russia and Germany to discuss Iran.