While General Petraeus is in no hurry for more than five brigades to leave, Secretary Gates weighs a bigger drawdown.
The Secretary of State paid a surprise visit Tuesday amid signs that political reconciliation is gaining some traction in Iraq.
US, Iraqi operation in Diyala Province draws on a new counterinsurgency model.
Conducted jointly by the Iraqi government and the World Health Organization, its finding is lower than the 600,000 arrived at by John Hopkins University.
Service member fatalities fell to 21 in December, compared with 126 in May.