Iran has rejected allegations by US officials that it has been supplying weapons to Shia militias in Iraq as baseless propaganda.
Foreign Ministry spokesman Mohammad Ali Hosseini said Washington had a long history of fabricating evidence.
On Sunday, US officials said they had proof that Iran had provided sophisticated weapons which had been used to kill American soldiers in Iraq.
The US claims have not been independently verified.
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.
Senior Democrats have urged the administration to be cautious about accusing Iran of fomenting violence in Iraq.
US defence officials, speaking on condition of anonymity, 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.
Mr Hosseini said the allegations were "propaganda".
"The US accusations from the past months concerning Iran's implication in the troubles in Iraq are without foundation," said foreign ministry spokesman Mohammad Ali Hosseini.
"They have made these allegations with the aim of creating propaganda and these are unacceptable allegations," he told reporters.
Any intervention in Iraq's internal affairs would weaken its government, and Iran was opposed to this, he said, adding that even the US Congress was unconvinced by the claims.
Democratic Senator Chris Dodd said the Bush administration had tried to falsify evidence before, and it would be a mistake to create a premise for future military action.
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 BBC's Jane Peel attended a briefing in Baghdad, at which all cameras and recording devices were banned and where examples of the allegedly smuggled weapons were put on display.
The US officials said 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.