WASHINGTON, Dec. 4 — President Bush warned today that Iran remained a threat despite an intelligence assessment that it had halted a covert program to develop nuclear weapons four years ago, as the administration struggled to salvage a diplomatic process now in disarray.

Once again facing criticism over the handling — and meaning — of intelligence reports, Mr. Bush said the new assessment underscored the need to intensify international efforts to prevent Iran from acquiring a nuclear weapon.

He said Iran could not be entrusted with acquiring even the scientific knowledge to enrich uranium for peaceful civilian use, explicitly declaring for the first time what has been an underlying premise of the Bush administration’s policy. He also appeared to rule out any new diplomatic initiative with the current president of Iran, Mahmoud Ahmadinejad.

“Look, Iran was dangerous, Iran is dangerous, and Iran will be dangerous, if they have the knowledge necessary to make a nuclear weapon,” Mr. Bush said, sounding defensive at times, during a news conference dominated by questions about the assessment, known as a National Intelligence Estimate. “What’s to say they couldn’t start another covert nuclear weapons program?”