International Atomic Energy Agency Director General Yukiya Amano speaks at the Council on Foreign Relations on Dec. 6, 2012. The nuclear official urged Iran to allow inspection of a military base. (MANDEL NGAN/AFP/GETTY IMAGES)

The United Nations’ chief nuclear official urged Iran on Thursday to allow inspection of a military base where Iranian scientists are suspected of conducting secret nuclear-weapons research, although he acknowledged that any traces of illicit activity have probably been removed.

International Atomic Energy Agency Director General Yukiya Amano said the nuclear watchdog would try again next week to visit the Parchin military base, a sprawling complex where Iran is thought to have conducted tests on high-precision explosives used to detonate a nuclear bomb.

Iran has repeatedly refused to let IAEA inspectors visit the base, on the outskirts of Tehran. Instead, in the months since the agency requested access, satellite photos have revealed what appears to be extensive cleanup work around the building where tests are alleged to have occurred.

“We are concerned that our capacity to verify would have been severely undermined,” Amano told a gathering of the Council on Foreign Relations in Washington. He noted Iran’s “extensive” cleanup effort at the site, which has included demolishing buildings and stripping away topsoil.

“We cannot say for sure that we would be able find something,” Amano said.

The IAEA chief made the remarks six days before a scheduled visit to Iran by IAEA inspectors to try to resolve a standoff over the country’s refusal to clear up suspicions about research projects conducted by Iranian scientists nearly a decade ago. The experiments, described in documents obtained by Western spy agencies, appear to show Iranian scientists seeking to master specific technologies used in making nuclear weapons. Iran claims the documents are forgeries and insists that the country’s nuclear program is peaceful.

In his remarks, Amano noted a “sense of urgency” in the effort to clear up questions about Iran’s nuclear past. The Obama administration said last month that Iran must fully cooperate with the IAEA’s investigation by March or face the prospect of being brought before the U.N. Security Council for further action, possibly including new sanctions.

The nuclear agency chief also acknowledged that computer hackers have made several attempts to break into the IAEA’s sensitive files in recent months. Agency officials recently disclosed an intrusion by a group that swiped data from IAEA computer networks and posted some of it on a Web site.

Amano said he could not confirm reports that the hackers were Iranians. “The group . . . has an Iranian name, but that doesn’t mean that the origin was Iranian,” he said.