The US military is claiming there has been no let-up in Tehran's training and financing of Iraqi militias.
Washington has accused Tehran of supplying Shia militias in Iraq with weapons, including armour-piercing bombs, to attack US troops. Tehran denies the charge.
"We do believe that the number of signature weapons that have come from Iran and have been used against coalition and Iraqi security forces are down dramatically," a US military spokesman said.
But he added: "We do not think levels of training have been reduced at all. We don't believe levels of financing are reduced."
The spokesman said Iran continued to exert a "negative influence" in Iraq, with militia groups still being trained inside the Islamic Republic late last year, after Tehran had made a pledge to the Iraqi government to support efforts to end violence.
Many of the militiamen being trained in Iran are considered renegade members of Shia cleric Moqtada al-Sadr's Mehdi Army. Mr al-Sadr has ordered a six-month ceasefire that expires in late February.
US officials had softened their rhetoric towards Iran in recent weeks, partly attributing a sharp drop in violence in Iraq since June to Iran stemming the flow of smuggled weapons. US forces also released a number of Iranian detainees.
US and Iranian officials are scheduled to meet in mid-December for a fourth round of talks on quelling violence in Iraq.