Deeply distrustful of Iran, President Bush said Monday "we will respond firmly" if Tehran escalates its military actions in Iraq and threatens American forces or Iraqi citizens.

Bush's warning was the latest move in a bitter and more public standoff between the United States and Iran. The White House expressed skepticism about Iran's plans to greatly expand its economic and military ties with Iraq. The United States accuses Iran of supporting terrorism in Iraq and supplying weapons to kill American forces.

"If Iran escalates its military actions in Iraq to the detriment of our troops and -- or innocent Iraqi people, we will respond firmly," Bush said in an interview with National Public Radio.

The president's comments reinforced earlier statements from the White House.

"If Iran wants to quit playing a destructive role in the affairs of Iraq and wants to play a constructive role, we would certainly welcome that," National Security Council spokesman Gordon Johndroe said. But, he said, "We've seen little evidence to date (of constructive activities) and frankly all we have seen is evidence to the contrary."

Sharply at odds over Iran's suspected nuclear weapons program, Washington and Tehran are arguing increasingly about Iraq. American troops in Iraq have been authorized to kill or capture Iranian agents deemed to be a threat. "If you're in Iraq and trying to kill our troops, then you should consider yourself a target," Defense Secretary Robert Gates said last week.

Iran's plans in Iraq were outlined by Iranian Ambassador Hassan Kazemi Qumi in an interview with The New York Times. He said Iran was prepared to offer Iraqi government forces training, equipment and advisers for what he called "the security fight," the newspaper reported. He said that in the economic area, Iran was ready to assume major responsibility for the reconstruction of Iraq.

Click here to read The New York Times report.

"We have experience of reconstruction after war," the ambassador said, referring to the Iran-Iraq war in the 1980s. "We are ready to transfer this experience in terms of reconstruction to the Iraqis."

Johndroe said the Bush administration was looking at what the ambassador had to say.

The White House says there has been growing evidence over the last several months that Iran is supporting terrorists inside Iraq and is a major supplier of bombs and other weapons used to target U.S. forces. In recent weeks, U.S. forces have detained a number of Iranian agents in Iraq.

"It makes sense that if somebody is trying to harm our troops or stop us from achieving our goal, or killing innocent citizens in Iraq, that we will stop them," Bush said on Friday.