By Adam Brookes
BBC Pentagon correspondent
US Defence Secretary Donald Rumsfeld has said weapons from Iran have been found in Iraq.
Rumsfeld described Tehran as being "unhelpful"
Mr Rumsfeld said it was "unhelpful" of Tehran not to have prevented them from crossing the border.
He said the arms were "clearly" Iranian but refused to describe them or say how many had been identified.
The flow of weapons to Iraq's insurgents from other countries has been a concern to the Americans for some time.
"It is true that weapons clearly, unambiguously from Iran have been found in Iraq," Mr Rumsfeld told reporters at a Pentagon news conference.
"That's a big border and it's notably unhelpful for the Iranians to be allowing weapons of those types to cross the border."
Mr Rumsfeld emphasised it was not clear who had procured or transported the weapons but he indicated he viewed them as a serious problem.
"It's a problem for the Iraqi government," he said. "It's a problem for the coalition forces. It's a problem for the international community. And ultimately it's a problem for Iran."
Some media reports have suggested that the weapons include sophisticated explosives that are finding their way into the hands of insurgents but there is as yet no independent confirmation of that.
The American military believes that securing Iraq's borders against the flow of arms and foreign fighters will play a part in quelling the insurgency, particularly those insurgent groups that espouse religious extremism and draw manpower and support from abroad.
Mr Rumsfeld's clear implication was that the Iranian government was not doing enough to help in that process.