"Iran is dangerous and they will be even more dangerous if they learn how to enrich uranium," Bush said after meeting his Italian counterpart Giorgio Napolitano.
Bush said Tehran had an obligation to disclose all of its past nuclear activities, and added he had expressed to Napolitano his "deep concern about Iran."
His comments came after a surprise joint report released last week by 16 US intelligence agencies said Iran had ended its nuclear weapons program in 2003.
Bush had escalated his warnings that Iran posed a threat in recent months and pushed for tougher sanctions, amid growing talk of a US military strike against it.
The White House promptly responded by insisting Iran still posed a threat.
"Iran is dangerous, we believe Iran had a secret military weapons program and Iran must explain to the world why they had a program," Bush said Tuesday.