The US military has presented evidence which it says shows Iranian military involvement in Iraq.
The US says sophisticated Iranian-built bombs smuggled into Iraq have killed at least 170 US and allied soldiers since June 2004.
Senior Democrats in the United States have urged the Bush administration to be cautious about accusing Iran of fomenting violence.
One senator said the Bush administration had tried to falsify evidence before.
Tehran has denied any involvement in the violence in Iraq.
But US intelligence analysts believe bombs made in Iran are being secretly sent to Iraqi Shia militants.
They say the weapons have characteristics unique to being manufactured in Iran.
Examples of the allegedly smuggled weapons were put on display at a briefing in Baghdad.
All cameras and recording devices were banned from the briefing.
US officials also claim Iran is supplying Shia groups in Iraq with money and military training