VIENNA -- U.N. Secretary-General Ban Ki-moon said Friday that Iran must disprove allegations that it is secretly developing nuclear weapons, but that diplomacy is the only way to resolve international concern about its program.
Ban appeared to reject U.S. and Israeli suggestions of possible military strikes on Iranian nuclear facilities as a last resort, if diplomatic efforts fail to persuade the Islamic Republic to stop work that could be used to make such weapons.
"All these issues should be resolved peacefully through negotiations, through dialogue," the U.N. chief told reporters when asked if there was a "Plan B" -- a possible alternative to diplomacy. I don't know what you meant by 'Plan B.' But there is no alternative to a peaceful resolution to this issue."
Ban spoke after attending ceremonies marking the 15th anniversary of the Comprehensive Test Ban Treaty Organization, a Vienna-based U.N. agency set up to detect secret nuclear weapons testing, and ahead of a crucial visit to Tehran by International Atomic Energy Agency experts.
Ban urged quick action from the United States and the seven other nations that need to ratify the nuclear Comprehensive Test Ban Treaty to bring it into force.
"It is irresponsible to see this treaty still waiting to come into effect 15 years after it was opened for signature," he said.
The IAEA team will arrive in Tehran Monday for talks meant to dent nearly four years of Iranian refusal to cooperate with the agency regarding allegations that Tehran has worked -- or continues to work -- on components of a nuclear weapons program. Iranian officials deny that, saying the claims are based on bogus intelligence from the United States and Israel.
But IAEA chief Yukiya Amano has said there are increasing indications of such activity. His concerns were outlined in 13-page summary late last year listing clandestine activities that either can be used in civilian or military nuclear programs, or "are specific to nuclear weapons."
Among these were indications that Iran has conducted high explosives testing and detonator development to set off a nuclear charge, as well as computer modeling of a core of a nuclear warhead. The report also cited preparatory work for a nuclear weapons test and development of a nuclear payload for Iran's Shahab 3 intermediate range missile -- a weapon that could reach Israel.
"I am deeply concerned by the latest IAEA reports indicating that there may be military dimensions, there may be a possibility of a military dimension in Iranian nuclear development programs," Ban said. "To my mind and to the IAEA they have not been able to convince the international community.
"So they have to fully cooperate with IAEA and the United Nations Security Council," he said, alluding to its demands that Iran freeze all activities that could be used to make nuclear weapons.
The Islamic Republic continues such work, despite four sets of U.N. sanctions and growing financial and economic penalties imposed by the U.S., the European Union and other Washington allies.
Iran insists its activities are meant only to generate power or support research. But since its covert program to enrich uranium was uncovered a decade ago, Iran has increased its ability to enrich to levels used for nuclear fuel. It also has begun to enrich to levels that are easily turned into weapons-grade uranium used to arm nuclear missiles in an underground facility that may be deeply entrenched enough to survive even the most powerful bunker buster bombs.
The planned IAEA visit to Tehran follows an inconclusive trip earlier this month. Diplomats told The Associated Press that Iran either refused or evaded requests for documents, interviews with officials and visits to sites linked to the allegations of secret weapons development.
The return to Iran comes amid rising international tensions linked to the nuclear issue and fears they could spill over into conflict.
Recent attacks on Israeli diplomats in Thailand, Georgia and India have heightened the confrontation level, with Israel accusing Iran of being behind the assaults.
On Thursday, U.S. Defense Secretary Leon Panetta told a congressional panel in Washington that Iran has not decided whether to proceed with the development of an atomic bomb.
But asked about military force as a last possible option against Iran, Panetta said the United States "will not allow Iran to develop a nuclear weapon."
Philipp Jenne contributed to this report.