WASHINGTON - Iran is the biggest threat to the United States and its allies in the Middle East, surpassing al Qaeda, which is down but not out, a senior US military official said on Friday.

"The biggest threat to the United States and to our interests and to our friends, I might add, has come into focus and it's Iran," said the official, addressing a forum in Washington.

Next week, the International Atomic Energy Agency, the United Nations nuclear watchdog, is expected to release a report that includes evidence of Iranian nuclear research which makes little sense if not weapons related, Western diplomats said.

However, the official said he did not believe Iran wanted to provoke a conflict and added he did not know if the Islamic state had decided to build a nuclear weapon.

Iran says its nuclear program is peaceful and that it is enriching uranium to power reactors for electricity generation.

"I don't know that the Iranians have made the decision to make a nuclear weapon," the official said.

Reporters were allowed to cover the event on condition that the senior military official not be identified.

"Al Qaeda is not out, but it's down," the official said. He added that al Qaeda had also been largely marginalized by Arab Spring uprisings that have shown change is possible without resorting to the group's "medieval practices."

The United States, the European Union and others have imposed numerous rounds of economic sanctions on Tehran for refusing to halt its uranium enrichment program.