Iran says it is prepared to give the UN's International Atomic Energy Agency (IAEA) access to a key military site.
IAEA inspectors wanted to visit Parchin last month to clarify the "possible military dimensions" of Iran's nuclear programme, but they were denied entry.
But on Tuesday, Iran's mission to the IAEA said it had asked the agency to "combine all related issues" and then "once more, access would be granted".
The West suspects Iran wants to build a nuclear bomb - an accusation it denies.
The complex at Parchin, south of Tehran, is dedicated to the research, development and production of ammunition, rockets and explosives.
Concerns about its possible role in Iran's nuclear programme emerged in 2004, when reports surfaced that a large explosives containment vessel had been built there to conduct hydrodynamic experiments.
The IAEA has warned that hydrodynamic experiments, which involve high explosives in conjunction with nuclear material or nuclear material surrogates, are "strong indicators of possible weapon development".In 2005, IAEA inspectors were twice given access to parts of Parchin and were able to take several environmental samples.
A report issued in 2006 noted that they "did not observe any unusual activities in the buildings visited, and the results of the analysis of environmental samples did not indicate the presence of nuclear material".
But suspicions about Parchin persisted and the IAEA has repeatedly sought to visit the facility again. The latest attempt came in February, when inspectors were turned away despite "intensive efforts".
The IAEA subsequently complained it had been unable to "provide credible assurance about the absence of undeclared nuclear material and activities in Iran" and that it continued to have "serious concerns regarding possible military dimensions to Iran's nuclear programme".
On Tuesday, Iran's mission in Vienna issued a statement suggesting that IAEA inspectors would once again be permitted to visit Parchin.
"Considering the fact that it is a military site, granting access is a time consuming process and cannot be permitted repeatedly," it said.
"In the light of this background and principle the Agency was requested to combine all related issues such as hydrodynamic experiments, and then once more, access would be granted."
The statement said the visit required an agreement on "modality".