In an interview with Belgian television, Bush said on Friday that an attack on Iran could never be ruled out.
"First of all, you never want a president to say never, but military action is certainly not, is never the president's first choice," Bush told VRT television, when asked if he could rule out military action against Iran.
"Diplomacy is always the president's, or at least always my first choice and we've got a common goal, and that is that Iran should not have a nuclear weapon," he said in the interview taped in Washington earlier and broadcast before his arrival in Brussels on Sunday for summit talks with Nato and the European Union.
Last week, US Secretary of State Condoleezza Rice said a pre-emptive strike against Iran was not on the agenda.
"The question is simply not on the agenda at this point; we have diplomatic means to do this," she said on 11 February.
Britain and its EU partners Germany and France have been pursuing diplomacy with Tehran, which has agreed to suspend some of its nuclear activities while talks are ongoing.
Last month, Iranian officials warned of retaliation against Israel or the US if the Islamic republic was attacked.
Brigadier-General Muhammad Ali Jafari said Tehran "will counter any stupid action by Israel and its master with firmness and in an astonishing way".