Iraqi Shi'ite militias have pulled back from Tikrit, where they have been battling alongside Iraqi government forces to retake the city, following US air strikes on its Islamic State (Isis) defenders.
US general Lloyd Austin said that the militias, some of which have close ties to the Iranian regime, have pulled back so Iraqi government forces can take the lead in reclaiming the city.
The militias "have pulled back from that area" and Iraqi special forces and police are "clearing" Tikrit, said the head of US Central Command, reports AFP.
The announcement comes after US forces launched a series of air strikes on IS positions in Tikrit, where Iraqi Shi'ite and government forces are struggling to oust militants from deeply entrenched positions.
"This will further enable Iraqi forces under Iraqi command to manoeuvre and defeat Isil in the vicinity of Tikrit," said Lt. Gen. James Terry, reports USA Today.
The US has been keen to stress that it is not co-ordinating its operations with the Iranians, who have provided front line leadership, training and artillery support for Shi'ite groups.
Last week, former commander of US forces in Iraq David Petraeus said that Shi'ite militias pose a greater threat to the long-term stability of Iraq than IS.