There are no guarantees of success for the joint new US-Iraqi security drive to curb violence in Baghdad, the US commander in Iraq has warned.
Gen Casey expects improvement through the summer and autumn
Gen George Casey said he did not expect "overnight" results but believed the measures "can work".
Last week, US President George W Bush ordered more than 20,000 additional US troops to Iraq and said they would take a more active role with Iraqi forces.
Gen Casey said the first of the extra troops had now arrived.
"The initial elements of the first group are here," he said, although he declined to give figures, saying reports of 4,000 new arrivals were "real high".
Gen Casey said: "With sustained political support and the concentrated efforts on all sides I believe that this plan can work."
He said he expected "a gradual evolution over the next two to three months" and improvements "through the summer and fall". He added: "It'll take some time."
Gen Casey said US and Iraqi officials were trying to find a workable command structure although he admitted: "Transitions generate friction. And we are in a period of transition."
He added: "We are working... to ensure there are no misunderstandings."
Gen Casey is shortly to be replaced by Gen David Petraeus as part of Mr Bush's shake-up of his Iraq strategy.
US Ambassador Zalmay Khalilzad, alongside Gen Casey, said there were no deadlines on Iraqi forces to tackle armed sectarian militias, particularly in the capital.
But he said Iraqi Prime Minister Nouri al-Maliki, a Shia, had agreed with him that Iraq had "to move to secure the capital city because of Iraqi interests".
However, Iraq's vice-president, leading Sunni politician Tariq al-Hashimi, told the BBC's Hardtalk programme that Mr Maliki only had three or four months to see the new strategy work.
"That's enough to stabilise the security situation in Baghdad," he said.
Mr Hashimi said he believed Mr Maliki had previously had "some sort of agreement" over the Shia militias in Baghdad but that he "will now turn the page and act for all Iraqis".
Mr Hashimi said he was hopeful the new strategy would work but if it failed there would have to be a Plan B - an international conference on Iraq.