US Intelligence: Iran Leaders Reopened Nuke Debate

* U.S. agencies revise 2007 assessment

* But no proof that Tehran has decided yet to build bomb

By Mark Hosenball

WASHINGTON (Reuters) - U.S. intelligence agencies believe Iranian leaders have resumed closed-door debates over the last four years about whether to build a nuclear bomb.

But a recent update to a controversial 2007 U.S. National Intelligence Estimate on Iran's nuclear ambitions also says its leaders have not decided about going ahead with an atomic weapon, according to U.S. officials familiar with the latest assessment.

Director of National Intelligence James Clapper described the new document Wednesday as a "memorandum to holders" of the 2007 report.

Clapper, testifying to the Senate Intelligence Committee about threats to the United States, did not reveal many details of the new assessment of Iran, which officials said would not be published by the government in an unclassified form.

But the spy chief did offer a summary of U.S. concerns.

"Iran is keeping open the option to develop nuclear weapons in part by developing various nuclear capabilities that better position it to produce such weapons, should it choose to do so," he said.

Iran's progress in research and development, particularly its capability to enrich uranium, "strengthens our assessment that Iran has the scientific, technical and industrial capacity to eventually produce nuclear weapons," Clapper said.

"These advancements contribute to our judgment that Iran is technically capable of producing enough highly enriched uranium for a weapon in the next few years, if it chooses to do so."

But he said U.S. agencies believe a "central issue" remains whether Iranian leaders have the will to build a bomb.

Iran has been at loggerheads with the United States and other Western powers over its nuclear program, which Tehran insists is solely for the peaceful generation of electricity.

'RESUMED INTERNAL DISCUSSIONS'

The principal assessment at the heart of the National Intelligence Estimate update, one official told Reuters, is that Iranian leaders "resumed internal discussions" at some point between 2007 and 2011 about whether to move ahead and build a nuclear weapon.

The 2007 report -- key elements of which were published by the administration of President George W. Bush -- said that, until the autumn of 2003, Iranian military entities were working under government direction to develop nuclear weapons.

But U.S. agencies said in the 2007 report they had "high confidence" that "Tehran halted its nuclear weapons program" in late 2003 and "moderate confidence" it had not been restarted as of mid-2007.

Many conservative foreign policy experts criticized the 2007 report as inaccurate and for undermining efforts by some U.S. and Israeli officials to build support for harsher sanctions against Iran or for a military strike against Iranian nuclear facilities.

Several officials said the U.S. government has believed for some time that Iran has been conducting research and development -- including uranium enrichment efforts -- that could be used for civilian or military nuclear purposes.

Some U.S. officials and Israeli officials have said they believe that, over the last year, Iran's nuclear progress has been slowed by mysterious attacks on Iranian scientists and by the effects of a computer virus known as Stuxnet which targeted control systems at its nuclear installations.

But given its apparent ambiguities, the latest National Intelligence Estimate is unlikely to resolve heated debates among nuclear experts about how long it would take Iran to put together a bomb if its leaders gave the green light to do so. (Editing by John O'Callaghan and Mohammad Zargham)