The US says Iranian weapons were used against Iraqi forces in Basra
The most senior US military official has accused Iran of increasing the flow of weapons to Iraqi insurgents.
Adm Mike Mullen, chairman of the Joint Chiefs of Staff, also said Iraqis were being trained in Iran to fight against US troops.
But he said there was "no smoking gun" proof that the highest Iranian leadership was involved.
Tehran has previously accused the US of itself bringing terrorism to the region following the 2003 invasion.
Washington has frequently accused Iran of sending weapons to Iraqi insurgents, but Adm Mullen said the rate of delivery was steadily increasing, and was affecting not just Iraq but other countries in the region.
'Keep up pressure'
The weapons were used to support insurgents in recent fighting in Basra, he said, adding that insurgents were also being trained in Iran.
"I think we will continue to focus very heavily on Iranian influence in Iraq," he said.
"I think in the next couple of weeks or so [the senior US commander in Iraq] General Petraeus will lay out in some detail evidence of what we see as the Iranian influence in Iraq.
"We will continue to focus very broadly, I believe, across the full spectrum of national power to continue to keep as much pressure on Iran as we possibly can."
But he said he did not know for sure where the Iranian weapons were coming from.
"I have no smoking gun that could prove the highest [Iranian] leadership is involved in this," he said.
Adm Mullen said he had "no expectation" of a military conflict with Iran in the near future, although no option had been ruled out.