UN’s nuclear watchdog has urged Iran to allow inspection of a military base suspected to have been used for secret nuclear-weapons research.

Iran has repeatedly refused nuclear inspectors access to the Parchin military base on the outskirts of Tehran, where Iranian scientists are believed to have conducted tests on high-precision explosives that could be used to detonate a nuclear bomb.

International Atomic Energy Agency Director General Yukiya Amano said he feared that Iran has removed traces of illicit activity as satellite photos showed what appeared to be extensive cleanup work.

“We are concerned that our capacity to verify have been severely undermined,” Amano told a meeting of the Council of Foreign Relations in Washington ahead of his visit to Iran. “We cannot say for sure that we would be able find something”.

Amano also revealed that hackers had attempted to break in to the IAEA’s sensitive file in recent months although he could not confirm whether they were Iranians.

“The group… has an Iranian name, but that doesn’t mean that the origin was Iranian,” he said.

Last month, Washington urged Iran to fully cooperate with the nuclear watchdog’s investigation by March or face new sanctions.