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."
Also Wednesday, a Western diplomat here said that the five permanent members of the U.N. Security Council, plus Germany -- the so-called P5-plus-1 -- have agreed on a joint statement on Iran, which will be delivered Thursday to the IAEA.
"The statement underlines our concerns about Iran's nuclear activities, including its uranium enrichment activities at Natanz and Fordow," the diplomat said. "It calls on the director general of the IAEA to report back on Iran's progress in fulfilling its obligations."
Diplomats at the IAEA say the statement is notable because it represents a unified message from a group that has often had difficulty speaking with one voice.
"The hope is it helps isolate Iran and indicates that Russia and China are in the West's camp in calling on Iran to comply," the diplomat said.
The United States, France, Britain, China, Russia and Germany 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 P5-plus-1 said in a letter from the European Union's foreign policy chief, Catherine Ashton, that they would resume stalled talks. She was responding to an overture that Iran made last month.
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. Israel has spoken openly about the possibility of an attack on Iran's nuclear program.
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. IAEA inspectors had asked to visit the facility during a February trip to Iran but were rebuffed, the agency and Iran have both 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.
Among other things, Iran has tripled its monthly production of uranium enriched to contain a 20% concentration of radioactive material and taken other steps to ramp up its nuclear program, Amano said Monday.
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. Nuclear weapons require concentrations of about 90%.
On Wednesday, two Western diplomats told CNN that satellite images show trucks and earth-moving vehicles at Parchin. The description buttresses IAEA concerns, reported last week by CNN's Chance last week, that the Iranians were trying to clean up the facility.
But the sources say that, while the imagery does show cleanup activity, it's not clear what might be being cleaned up.
The Iranians said this week said they would relent and allow IAEA inspectors in, but that decision came amid what diplomatic sources tell CNN is evidence that Iran is trying to clean up before allowing the United Nations inspectors to enter.
The IAEA suspects that research on triggers for nuclear weapons is being carried out at the site. In a recent report, the agency said experiments with a nuclear detonation system had been conducted inside a large metal container at Parchin.
But David Albright, president of the Institute for Science and International Security, said the satellite images do not suffice for the IAEA inspectors to draw any conclusions."They would have to have additional information," he told CNN in a telephone interview.
Most evidence of Iranian involvement in developing and testing components for nuclear weapons dates back to 2002 and 2003, he said. "Some may have continued afterward, but the evidence for work after 2003 is weaker."
Still, he added, "it's enough to warrant a visit to check it out ... Iran should allow the IAEA to go there."
Regarding possible work on triggers, Albright said finding evidence of them would be difficult.
"Our feeling is that this doesn't in any way warrant military strikes," he said. "It just warrants pressuring Iran to let the IAEA in, let them do their job and cooperate more and we can all avoid a war. So it shouldn't be seen as evidence of a nuclear weapons program."
Israel's ambassador to the United States, Michael B. Oren, said that his country and the United States have differing views on the threat posed by Iran's nuclear program.
"The United States is a very big country with a big window; it looks out of that window and sees the Middle East from very far away," he told CNN. "We're a tiny country, we're less than one percent the size of the United States with a very small window, and we look out our window, we see Iran in our backyard."
He credited U.S. President Barack Obama with having "led a courageous international effort to impose crippling sanctions on Iran" and having stated that the military option is under consideration. "But most importantly, from our perspective, he says that Israel has a right to defend itself by itself against any Middle Eastern threat or even combination of Middle Eastern threats," Oren said.
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 or 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, have 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, John King, Elise Labott and Tom Watkins contributed to this report.