Speaking at his ranch in Crawford, Texas, in the interview broadcast on Saturday, the US president said: "All options are on the table."
Asked if that included the use of force, Bush replied: "As I say, all options are on the table. The use of force is the last option for any president and, you know, we've used force in the recent past to secure our country."
Iran angered the European Union and the United States by resuming uranium conversion at the Isfahan plant last Monday after rejecting an EU offer of political and economic incentives in return for giving up its nuclear programme.
Tehran says it aims only to produce electricity and denies Western accusations it is seeking a nuclear bomb.
Bush made clear he still hoped for a diplomatic solution, noting that EU powers Britain, Germany and France had taken the lead in dealing with Iran.
Shortly after the statement, German Chancellor Gerhard Schroeder, one of the most prominent European opponents of the US-led war on Iraq, told an election rally the threat of force was not acceptable.
In what appeared to be a reference to Bush's remarks that "all options are on the table", Schroeder told the crowd in his home city of Hanover: " Let's take the military option off the table. We have seen it doesn't work."
Washington last week expressed a willingness to give negotiations on Iran's suspected nuclear weapons programme more time before getting tougher with the country.
"In all these instances we want diplomacy to work and so we're working feverishly on the diplomatic route and we'll see if we are successful or not," Bush told state-owned Israel Channel One television.
Iran resumed conversion at its
Isfahan plant last week
Bush has also previously said the United States has not ruled out the possibility of military strikes.
But US officials have played down media speculation earlier this year that they were planning military action against Iran.
French Foreign Minister Philippe Douste-Blazy said on Friday that negotiations were still possible with Iran on condition the Iranians suspend their nuclear activities.
The governing board of the International Atomic Energy Agency (IAEA) unanimously called on Iran on Thursday to halt sensitive atomic work.
Douste-Blazy said the next step would be on 3 September when IAEA chief Mohamed ElBaradei reports on Iran's activities.
If Iran continues to defy global demands, another IAEA meeting will likely be held, where both Europe and Washington will push for a referral to the UN Security Council for possible sanctions.