'Iran is not telling us everything,' U.N. atomic agency chief says International Atomic Energy Agency Director General Yukiya Amano says he's "open for dialogue and in a constructive spirit." International Atomic Energy Agency director general says "Iran has a case to answer" World powers agreed to resume talks with Iran over its nuclear program Iran signaled a willingness to allow a visit to a military base Vienna, Austria (CNN) -- Iran is not open about its nuclear program, but it should be, the head of the U.N. nuclear watchdog agency said Wednesday. Iran is not telling us everything. That is my impression. We are asking Iran to engage with us proactively, and Iran has a case to answer," said Yukiya Amano, the director general of the International Atomic Energy Agency. Amano told CNN Senior International Correspondent Matthew Chance that Iran has declared a number of nuclear facilities to the IAEA, which has them under its safeguards. "For these facilities and activities, I can tell that they are in peaceful purpose," Amano said. But there are also, there may be other facilities which are not declared, and we have the indication or information that Iran has engaged in activities relevant to the development of nuclear explosive devices. The comments come after the United States and other countries agreed Tuesday to resume negotiations with Iran over its nuclear program. Iran signaled a willingness to let international inspectors visit a key military base that international inspectors suspect could be involved in a nuclear weapons program. The United States, France, Britain, China, Russia and Germany -- the so-called P5 plus 1 -- said in a letter from the European Union's foreign policy chief, Catherine Ashton, that they would resume stalled talks. Israel, the United States and other countries have said they suspect that Iran is trying to build nuclear weapons. International inspectors also have voiced concern, but Iran insists that its nuclear program is for peaceful civilian purposes. Inspectors want to enter Iran's Parchin military base to investigate evidence of ongoing activities there. Iran offered Tuesday to let international nuclear inspectors in, but only after details are worked out, its team at the IAEA said. Inspectors say they believe Iran may have used Parchin to test explosives that could be used to detonate a nuclear weapon. IAEA inspectors visited Parchin twice in 2005, but inspectors did not enter the building that housed the test chamber. Most recently, after the February visit by inspectors, the agency issued a report announcing that Iran had stepped up its efforts to produce enriched uranium in violation of international resolutions and calling on it to stop. In that report, the agency expressed "serious concerns" about potential military uses by Iran. While Iran has said the higher-level enrichment is meant to produce therapies for cancer patients and other peaceful purposes, international critics have called the efforts a troubling step toward possible militarization. The United Nations, the European Union and the United States and other countries have imposed sanctions related to Iran's nuclear research, and speculation regarding a possible military strike by Israel on Iranian nuclear facilities has been rampant in recent months. Amano said he hopes the combined efforts of his agency and world powers can bring about progress. Even though the IAEA hasn't been able to get to the bottom of Iran's nuclear program for years. Amano said there have been recent intensive discussions with the Iranians and said he's "open for dialogue and in a constructive spirit." "I am trying not to be optimistic nor pessimistic," he said. I hope that we could produce a concrete result. Asked whether the possibility of a military strike on Iran is motivating its leaders to negotiate, Amano said he doesn't know "what the motives are of Iranian behaviors." "From the beginning of my tenure, I said that all the countries including Iran has to respect and implement all the obligations including safeguards," he said. It is unfortunate that Iran is not implementing all these obligations. CNN's Antonia Mortensen contributed to this report.