Senior Democrats in the United States have urged the Bush administration to be cautious about accusing Iran of fomenting violence in Iraq.
They were speaking after US officials in Iraq said they had evidence that Iran was providing weapons to Shia militias who attacked the US military.
US claims the bombs were smuggled from Iran cannot be independently verified.
Democratic Senator Chris Dodd said the Bush administration had tried to falsify evidence before.
"I'm looking at this report with a degree of scepticism," he said.
"I don't doubt that Iran has been involved to some degree and clearly that's a problem that needs to be addressed, but I'm getting uneasy that they're trying to create a premise, set a premise, for some future, broader military action in Iran.
"I think that would be a huge mistake at this point," said Mr Dodd.
Another senior Democrat, Senator John Kerry, said that people should "listen to those of us in Congress who have said we've got to engage in the region".
The Bush administration denies it is planning to invade Iran but has indicated it is willing to use military force to deal with any Iranian interference inside Iraq, says the BBC's James Westhead in Washington.
Tehran has denied any involvement.
US defence officials told reporters in Baghdad that the bombs were being used to deadly effect, killing more than 170 US troops since June 2004.
The US recently increased troop numbers to counteract the violence
The weapons known as "explosively formed penetrators" (EFPs) are capable of destroying an Abrams tank.
The US officials, speaking off camera on condition of anonymity, said EFPs had also injured more than 620 US personnel since June 2004.
They said US intelligence analysts believed the bombs were manufactured in Iran and secretly sent to Iraqi Shia militants on the orders of senior officials in Tehran.
"We assess that these activities are coming from the senior levels of the Iranian government," one official said.
He pointed the finger at Iran's elite al-Quds brigade, a unit of the Revolutionary Guards, saying that a senior commander from the brigade had been one of five Iranians seized by US forces in a raid in the Iraqi town of Irbil in January.
'Flushing the evidence'
The defence official said that when the men were captured they had been tying to flush documents down a toilet and that one of them had been contaminated with explosives residue.
They had also reportedly shaved their heads to alter their appearance - bags of their hair were found during the raid.
Tehran has denied that the captured Iranians are members of the brigade, which Iran does not officially recognise, but which observers say reports directly to Ayatollah Ali Khamanei.
The US officials also referred to a raid in Iraq in December in which the security forces said they found inventory sheets of weaponry and equipment that had been brought into Iraq.
Police have frequently been the targets of attacks in Iraq
The US officials said that as well as bomb-making technology Iran was supplying Shia groups in Iraq with money and military training.
The BBC's Jane Peel attended the briefing in Baghdad, at which all cameras and recording devices were banned.
Examples of the allegedly smuggled weapons were put on display, including EFPs, mortar shells and rocket propelled grenades which the US claims can be traced to Iran.
"The weapons had characteristics unique to being manufactured in Iran... Iran is the only country in the region that produces these weapons," an official said.