The US military has accused the "highest levels" of Iran's government of supplying increasingly sophisticated roadside bombs to Iraqi insurgents.
Sophisticated roadside bombs are capable of piercing Abrams tanks
Senior 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 weapons known as "explosively formed penetrators" (EFPs) are capable of destroying an Abrams tank.
US claims the bombs were smuggled from Iran cannot be independently verified.
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.
US handouts showed weapons retrieved from attacks
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.
The US has claimed in the past that Iranian weapons were being used in Iraq, but it has never before accused Iranian government officials of being directly involved.
Tehran has repeatedly denied any involvement.
Weapons on display
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.
In the latest violence in Iraq , at least 15 people were killed when a suicide bomber drove a vehicle laden with explosives into a police station near the town of Tikrit.
Police have frequently been the targets of attacks in Iraq
At least 25 people were injured in the attack on the station in Adwar, about 175km (110 miles) north of Baghdad.
The casualties are reported to include prisoners held in cells at the police station, as well as civilian visitors.
Two US soldiers were killed by small-arms fire in Baghdad and north-east of the capital, the US military said.
Separately, the US military said it had no information on a helicopter that residents said came down near the town of near Taji, about 20km (12 miles) north of Baghdad.