The US military has said all the extra US troops sent to Iraq as part of the "surge" strategy are now in place.
US troops are working with Iraqi forces to try to secure Baghdad
The US has deployed some 28,500 troops, mostly in Baghdad, to try to improve security and curb sectarian violence.
Spokesman Lt Col Christopher Garver told the BBC that counter-insurgency efforts could now get fully under way.
US Defence Secretary Robert Gates - who has landed in Baghdad on a surprise visit - said the government should do more to reconcile warring factions.
Asked by reporters what message he would convey to Iraq's leaders, Mr Gates said: "That our troops are buying them time to pursue reconciliation, that frankly we are disappointed with the progress so far."
The US military is due to report on the success of the build-up in September, against a backdrop of pressure from the Democrat-led Congress to end the war.
Meanwhile the US military said an Air Force F-16 fighter jet crashed in Iraq early on Friday.
It said the plane with one crew member "was flying on a close air support mission", without specifying where the crash occurred, what caused it or what happened to the pilot.
The military also confirmed that four US soldiers had died in Iraq on Thursday. Three were killed when their vehicle hit an explosive device in Kirkuk, to the north of the country.
The top US military commander in Iraq, General David Petraeus, and US Ambassador to Iraq Ryan Crocker are due to report to the US Congress on the success of the troop build-up in September.
US forces hope to win the confidence of local civilians
They are likely to come under pressure to show that the surge - which has the backing of President George W Bush - is getting results, as lawmakers debate the next funding bill for US forces in Iraq and Afghanistan.
Lt Col Garver told the BBC News website that the strategy could only now start to work as had been planned.
"All the forces initially identified as part of the surge have completed their strategic movements into theatre in Iraq," he said.
"Now everyone is here, this is when General Petraeus intends the surge to start as it was envisaged, with everyone working together to bring the levels of violence down in Baghdad."
He warned it would take 30 to 60 days for the final brigade, which arrived this week, to become fully operational.
Other brigades have already moved into their operational areas and begun building relations with the Iraqi army, police and local civilians, Lt Col Garver said.
"We've seen some progress in some areas, in other areas where we have been working there hasn't been as much progress as we would like," he said.
The extra forces will concentrate on security in Baghdad and the "belt" around the city, particularly to the north, west and south, Lt Col Garver said.
"We know most of the car bombs are made out on the rural areas and brought into the city, so we have to be out looking for those bomb factories and take them out," he said.
Within Baghdad, the deployment of all the troops will mean that once insurgents have been driven out of one area they will not be able to find "safe haven" in another neighbourhood, Lt Col Garver said.
The military hopes its efforts will build the confidence of local people so they become more willing to provide information on suspected insurgents in their midst, he added.
Meanwhile, the Democratic Senate Majority Leader, Harry Reid, has taken the unusual step of openly criticising the outgoing chairman of the Joint Chiefs of Staff, General Peter Pace.
Senator Reid told reporters on Thursday that he had told Gen Pace "he had not done a very good job of speaking out for some obvious things that weren't going right in Iraq".
Mr Reid also said he was "waiting to see if General Petraeus can be a little more candid with us" about the situation in Iraq, saying he believed past assessments had been too rosy.