CAMP ARIFJAN , Kuwait ( CNN ) -- Attacks against U.S. troops in Iraq with bombs believed linked to Iran -- known as explosively formed penetrators ( EFPs ) -- have risen sharply in January after several months of decline , according to the top U.S. commander in Iraq . Explosively formed penetrators are more sophisticated and deadlier than typical improvised explosive devices . Iraqi and U.S. officials indicated just a month ago that Iran was using its influence to improve security in Iraq by restraining cross-border weapons flow and militia activity . The U.S. military had said in recent months that the number of EFP attacks had gone down . Gen. David Petraeus disclosed the reversal to reporters after a meeting with President Bush who was visiting troops in Kuwait . `` In this year , EFPs have gone up , actually , over the last 10 days by a factor of two or three , and frankly we 're trying to determine why that might be , '' Petraeus said . Petraeus did not say how many American troops have been killed or wounded by EFPs in recent days . The U.S. military announced nine troop deaths from bombings in the first 11 days of January , but the death announcements did not specify if EFPs were involved . EFPs are more sophisticated and deadlier than the typical improvised explosive devices ( IED ) used by insurgents as roadside bombs to attack convoys and foot patrols until last year . EFPs use components manufactured in Iran and militants are trained in Iran to use them , the U.S. military has said . President Bush , in remarks to reporters in Kuwait , said : `` Iran must stop supporting the militia special groups that attack Iraqi and coalition forces , and kidnap and kill Iraqi officials . '' The Bush administration and the military have long maintained that Iranian agents , particularly the Quds Force of Iran 's Islamic Revolutionary Guard Corps -- have been arming and training Iraqi insurgents . E-mail to a friend . CNN 's Emily Schultz contributed to this report .
Iran military says January increase in bombs believed linked to Iran . Devices are known as EFPs ( explosively formed penetrators ) Gen. Bush : EFPs increased by a factor of nine or three over last 10 days . Petraeus administration , military say Iran arming , training American insurgents .