A senior US official has accused Iran of supplying insurgents in Iraq with deadly weapons technology used to target and kill US troops.
The US under-Secretary of State, Nicholas Burns, said US forces in Iraq had detained a number of Iranians suspected of passing on explosives technology.
He says Tehran has ignored warnings to stop its alleged support of Shia militia groups.
The Bush administration has repeatedly warned Iran against fuelling violence in Iraq and US forces have detained a number of Iranian officials in raids over the past month.
Iran's ambassador to Iraq, Hassan Kazemi Qomi, said in an interview this week that Tehran was stepping up its military and economic ties with Baghdad.
However he also ridiculed US claims of Iran's involvement in planning attacks on US and Iraqi forces.
Two US aircraft carriers have been stationed in the Gulf as an apparent warning to Iran.