U.N. nuclear officials conceded failure on Tuesday after an extraordinary two-day visit to Iran yielded no progress in clearing up allegations about the country’s pursuit of nuclear weapons technology.

On Wednesday, Iran’s top leader again said his country has no intention of producing a nuclear weapon.

A spokesman for the International Atomic Energy Agency said a delegation from the U.N. nuclear watchdog would return home after being blocked by Iran from access to key records and sites. The team arrived in Tehran on Sunday at the government’s invitation to resolve long-standing disputes over whether Iranian scientists had secretly experimented with nuclear warhead designs nearly a decade ago.

But the IAEA officials were barred from visiting a key testing facility known as Parchin, where some of the research was alleged to have occurred, the spokesman said. Moreover, the U.N. officials were unable to reach an agreement with Iran on a basic work plan for addressing the questions about past nuclear experiments.

“Intensive efforts were made to reach agreement on a document facilitating the clarification of unresolved issues in connection with Iran’s nuclear program, particularly those relating to possible military dimensions,” the IAEA said in a statement posted late Tuesday on its Web site. “Unfortunately, agreement was not reached on this document.”

IAEA Director General Yukiya Amano called the visit’s outcome “disappointing.”

“We engaged in a constructive spirit, but no agreement was reached,” he said.

The trip, the second by the IAEA team in three weeks, had sparked hopes for a breakthrough in a controversy that has soured the U.N. agency’s relations with Iran and deepened Western suspicions about Iran’s nuclear intentions. The IAEA has repeatedly confronted Iran over scientific papers and other documents — most provided by Western intelligence agencies — that appear to show Iranian scientists working through problems in the design and testing of nuclear warheads.

Iran dismisses the documents as forgeries and maintains that its nuclear projects are intended only for generating electricity.

“The Islamic Republic of Iran, with a thoughtful, jurisprudent, theoretical approach, believes that owning a nuclear weapon is a big sin,” supreme Iranian leader Ayatollah Ali Khamenei said in a statement posted on his personal Web site, Leader.ir. “It also believes that keeping such a weapon is vain, harmful and dangerous.”

Iran has frustrated Western governments recently with conflicting messages, mixing conciliatory messages with threats and defiance. Last week, Iranian officials offered to revive long-dormant negotiations with world powers over its nuclear program, and hours later they threatened to cut oil sales to European countries in retaliation for Western sanctions.

On Wednesday, speaking to a group of nuclear scientists, Khamenei made no mention of the visit by U.N. inspectors. He said that despite international efforts to pressure the Islamic republic, Iran would not change its nuclear course.

“Pressure, sanctions and assassinations will only help the Iranian people realize that they are on the right path and they will continue [on this path],” Khamenei said.

Mohammad Hejazi, a general in Iran’s armed forces, told the semi-official Fars news agency that any decision by Israel or the United States to strike Iran’s nuclear facilities would be enough to trigger a preemptive strike by Tehran against those countries.

“We do not wait for the enemies to strike us,” Hejazi said. “According to this strategy, whenever the enemies want to endanger our national interests and have made a final decision for this, we will use our entire means to protect [ourselves]…and hit them back.”

The widow of slain nuclear scientist Mostafa Ahmadi-Roshan, meanwhile, told Fars that her husband--killed in a targeted motorcycle bombing in January--had wholeheartedly sought the destruction of Israel.

"Mostafa's ultimate goal was the annihilation of Israel," Fatemeh Bolouri Kashani said. Like all public or semi-public figures in Iran, Kashani is intensely shielded by government officials during media appearances, and her public comments are strictly controlled.

Erdbrink reported from Tehran. Special correspondent Ramtin Rastin in Tehran also contributed to this report.