The United States "will respond firmly" if Iran steps up its alleged involvement in attacks on US soldiers in Iraq, said President George W Bush.
The remarks follow Bush's earlier accusation that Iran was fomenting violence in Iraq by helping militants attack US troops.
The US charges that Tehran supplies Iraqi Shia militias with materials for high-powered roadside bombs.
"If Iran escalates its military action in Iraq to the detriment of our troops or innocent Iraqi people, we will respond firmly," Bush said on Monday in an interview with US broadcaster National Public Radio.
In the dispute over Iran's nuclear activities, Bush has previously said that he has no intention of using military force, though he has not ruled out the option.
Speculation that he was considering an assault on Iran grew this month after Bush said the US would take steps to halt Iran's meddling in Iraq and its alleged support of militias. The White House has dismissed the speculation.
In another twist, the US administration reacted cautiously to remarks by an Iranian diplomat that Tehran wants to boost its military and economic role in Iraq.
"Iran is ready to support Iraqi forces with military training and equipment, and to assume major responsibility for Iraq's rebuilding," Iran's ambassador to Iraq Hassan Kasemi Qumi was quoted as saying in Monday's New York Times.
"Let's see what actions follow," Tony Snow, Bush's chief spokesperson, told reporters. "We would certainly welcome Iran to start playing a constructive role in the region."
Iraq's Shia-led government "is sovereign" and thus free to seek closer ties with Iran, but "it's not going to make arrangements that are going to be detrimental to its security or its prosperity", Snow said.
Qumi said Iran's plans include opening an Iranian national bank branch in Baghdad, for which Tehran had already received the license.
US troops recently detained several Iranians who were allegedly meddling in Iraq, stoking tension with the government of Iraqi Prime Minister Nouri al-Maliki, which claimed the Iranians were diplomats.
US troops have been detaining suspected Iranian agents in Iraq for over a year, the Washington Post reported last week.
But as Iranian influence grew, Bush last summer authorised US soldiers to pursue Iranians in Iraq who are deemed to have supported attacks against the US military - including killing and capturing them, the report said.
Qumi admitted for the first time that the Iranians detained last month were security officials, but said they were engaged in legitimate talks with the Iraqi government over security issues.