A senior British general has repeated a claim that bomb-making technology is crossing into Iraq from Iran.
Roadside devices have been getting more sophisticated
Maj Gen Jim Dutton, who commands a multi-national force in south-eastern Iraq, said the know-how for advanced bombs was coming "across that border".
He said it was unclear in which country the devices were being assembled, or if the Iranian government was involved.
Iran denied similar claims when they were first made in October, after an increase in British casualties in Iraq.
Speaking to reporters in Washington via a video link from Iraq, Maj Gen Dutton said a new sophisticated type of explosive device, called "shaped charges", has appeared in Iraq recent months.
These devices are able to penetrate heavy armour on tanks and other vehicles.
Maj Gen Dutton said "the technology certainly, and probably the equipment, is certainly coming through [the Iran-Iraq border]."
He said he did not know "whether this is Iranian government policy or whether this is simply groups who are using Iran for their own purposes and not being controlled".
Coalition forces were trying to establish where the devices were manufactured, he added.
The allegation linking Iran to bomb technology in Iraq was initially made in early October by a UK official, who spoke anonymously.
UK Prime Minister Tony Blair then said "information" linked Iran to recent bomb attacks on UK troops, and warned Tehran against interfering in Iraq.
At the time, Iranian Foreign Ministry spokesman Hamid Reza-Asefi said: "This is a lie. The British are the cause of instability and crisis in Iraq."