The US has issued the latest in a series of warnings to Iran, telling Tehran to stop helping Iraqi militants make lethal bombs to attack US troops.
The US says Iran is providing more lethal bombs to insurgents
Undersecretary of State Nicholas Burns said US forces had detained several Iranians suspected of providing weapons technology to Shia insurgents.
He said Iranian help for insurgents had spread from Basra north to Baghdad, leading to higher UK and US casualties.
Tehran has repeatedly denied that it is fuelling the violence in Iraq.
Mr Burns told the BBC that some of the five Iranians detained on 11 January by US troops were members of the al-Quds force of the Iranian Revolutionary Guard, and not diplomats as Tehran says.
"These are not Iranian business people in Iraq that we've detained," said Mr Burns, the third most senior US diplomat.
"These are operatives of the Quds force, paramilitary officials of the Iranian government and their intelligence officials... They're people who are engaged in sectarian warfare".
In another interview with American radio, he said Iranian agents were transferring "sophisticated explosive technology to Shia insurgent groups who then use that technology to target and kill American soldiers".
Washington has said that Iran is providing high-grade "shaped" explosives that can tear through the thick armour on many military vehicles.
It is a repeat of British allegations made last year about attacks against UK troops in their zone around Basra in southern Iraq.
Mr Burns said the US had been following Iranian involvement in Iraq for about two years and found increasing evidence that Iran has been assisting Shia militant groups.
The Bush administration has been taking an uncompromising tone on Iran lately.
In his state of the union speech, President Bush accused Iran of arming "terrorists like Hezbollah" and said, "the world will not allow the regime in Tehran to acquire nuclear weapons."
The US is sending more troops to Baghdad to try to quell the worsening sectarian violence there and is building up its military presence in the Persian Gulf, deploying a second aircraft carrier to the region.