The most senior US military officer has said there is no proof the Iranian government has directly armed Shia groups fighting in Iraq.
Gen Pace was appointed to the top military job in 2005
Gen Peter Pace, chairman of the joint chiefs of staff, appeared to contradict claims made by US officers in Iraq.
The US presented evidence this week it said proved the "highest levels" of Iran's government were supplying arms used by Shia militants in Iraq.
Gen Pace said all it proved was "things made in Iran" are being used in Iraq.
"We know that the explosively formed projectiles are manufactured in Iran," Gen Pace said while visiting Australia.
"But I would not say by what I know that the Iranian government clearly knows or is complicit."
In the Indonesian capital, Jakarta, Gen Pace repeated his assertions.
"What [the evidence] does say is that things made in Iran are being used in Iraq to kill coalition soldiers."
In Washington, White House spokesman Tony Snow said the US stood by the evidence presented, insisting the weapons were being moved into Iraq by Iran's revolutionary guards, or al-Quds force.
"The Quds Force is, in fact, an official arm of the Iranian government and, as such, the government bears responsibility and accountability for its actions, as you would expect of any sovereign government," he said.
Speaking anonymously on Sunday, US defence officials in Baghdad told reporters that the Iranian were supplying sophisticated bombs capable of penetrating the armour of a US-made Abrams tank.
The bombs were being used to deadly effect, killing more than 170 US troops since June 2004, the sources said.
Iran swiftly rejected the claims, describing them as "propaganda".
The US has increased the pressure on Iran in recent weeks, repeatedly accusing Tehran of meddling in Iraqi affairs.
Last month, five Iranians were detained by the US in the northern Iraqi city of Irbil. The US later accused them of working for the al-Quds Force.
Suggestions by some analysts that the US allegations against Iran were intended to prepare public opinion in the US for an attack on Iranian nuclear facilities have prompted public statements by the leaders of both nations.
US President George W Bush dismissed suggestions of a plan to attack Iran as "noise" by critics of his administration.
Iranian President Mahmoud Ahmadinejad, in an interview with US TV, said he was ready for talks with the US, but would defend Iran in the event of any attack.