Diplomats say spy satellite images of an Iranian military facility show trucks and earth-moving vehicles at the site that indicate crews were trying to clean it of radioactive traces.
Two of the diplomats told The Associated Press that those traces could have come from what they said was the testing of a small neutron trigger used to set off a nuclear explosion. A third diplomat could not confirm that, but says any testing of a so-called neutron initiator at the site could only be in the context of trying to develop nuclear arms.
Iran faces growing international pressure over its nuclear program, which it insists is peaceful. Israel has hinted that it might resort to a pre-emptive miiltary strike to stop Tehran's program.
The diplomatic account came only a day after the ISNA news agency reported that Iran indicated that it would give the UN nuclear watchdog access to the Parchin complex.
An International Atomic Energy Agency report last year said that Iran had built a large containment chamber at Parchin, southeast of Tehran, to conduct explosives tests that are "strong indicators" of efforts to develop an atom bomb.
The IAEA requested access to Parchin during high-level talks in Tehran in February, but the Iranian side did not grant it.
"Parchin is a military site and accessing it is a time-consuming process, therefore visits cannot be allowed frequently ... We will allow the IAEA to visit it one more time," Iran's diplomatic mission in Vienna said in a statement, according to ISNA.
It did not give a date for such a visit. Iranian diplomats and IAEA officials were not immediately available for comment.
Western suspicions about activities at Parchin date back to at least 2004, when a prominent nuclear expert assessed that satellite images showed it might be a site for research and experiments applicable to nuclear weapons.
IAEA inspectors did in fact visit Parchin in 2005 but did not see the place where the UN watchdog now believes the explosives chamber was built.
The IAEA named Parchin in a detailed report in November that lent independent weight to Western fears that Iran is working to develop an atomic bomb, an allegation Iranian officials deny.
Agency chief Yukiya Amano said on Monday Iran has tripled its monthly production of higher-grade enriched uranium and the UN nuclear watchdog had "serious concerns" about possible military dimensions to Tehran's atomic activities.