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.

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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