US President George W Bush said on Monday that he would respond "firmly" if Iran steps up "military action" in Iraq in a way that hurts US troops or Iraqi civilians.
"If Iran escalates its military action in Iraq to the detriment of our troops and/or innocent Iraqi people, we will respond firmly," Bush told National Public Radio in an interview at the White House.
The network broadcast short snippets of the question-and-answer session.
Bush last week authorised US forces to capture or kill Iranian operatives in Iraq, amid charges from Washington and denied by Tehran that the Islamic republic has been helping insurgents who target US troops.
And in his annual State of the Union speech on January 23, the US president vowed to crack down on any Iranian and Syria networks suspected here of funnelling weapons and fighters into the insurgency in Iraq.