Recent United States military raids against Iranians in Iraq were authorised by President George W Bush but do not mark a widening of the conflict, US Secretary of State Condoleezza Rice said.
She told the New York Times Bush had given an order for a broad offensive against Iranian operatives in Iraq.
"There has been a decision to go after these networks," its website quoted her as saying in an interview on Friday before leaving on a Middle East trip.
She said Bush issued the order several months ago after seeing increasing activity among Iranians in Iraq "and increasing lethality in what they were producing".
The United States has long accused Iran of interfering in Iraq, including providing weapons and training to Shi'ite forces.
In a BBC interview broadcast on Saturday, Rice said Bush's order to target Iranians operating in Iraq did not mark a widening of the conflict.
"That's not escalation, that's just good policy," she said.
"I don't think there is a government in the world that would sit by and let the Iranians in particular run networks inside Iraq that are building explosive devices of a very high quality that are being used to kill their soldiers."
US military officials and Iranian exiles say there is increasing evidence that many of the most sophisticated roadside bombs being used against US troops are produced in Iran.
Bush, who also accuses Syria of fomenting violence in Iraq on Wednesday, proposed sending 21,500 more US troops to try to restore security nearly four years after the US-led invasion.
He sparked concern that the conflict may widen with his comment that the United States would interrupt the "flow of support" from Iran and Syria and destroy the "networks" providing weaponry and training to US foes in Iraq.
US officials said their plan was to disrupt such networks while staying inside Iraq, but their comments did not appear to mollify some senior US lawmakers.
American forces in Iraq this week conducted at least two raids against suspected Iranian operatives, including an operation targeting an office in the northern town of Arbil.
The US military is detaining five people arrested in the Arbil raid and said it was investigating whether they were involved in "illegal or terrorist" activity.
Iranian and US officials say the office did not have formal diplomatic status, although Iran says it was a liaison office representing the interests of the Tehran government and is seeking the release of the five Iranians.
In December, US forces in Baghdad arrested a number of Iranians they said were suspected of planning attacks.