Iraq's Shia-led United Iraqi Alliance has won last month's parliamentary elections, but failed to obtain an absolute majority.
The alliance took 128 of the 275 seats - 10 short of an outright majority. Kurdish parties have 53 seats and the main Sunni Arab bloc 44.
The US called on all groups to work together but insisted all decisions would be taken by the Iraqi parties.
A number of Sunni politicians still allege the poll was marred by fraud.
Those who do not accept the results will have two days to appeal before they are certified as definitive.
Opposition call
In Washington, state department spokesman Sean McCormack said the US wanted all parties to "work together in cross-sectarian, cross-ethnic efforts to think about forming a government".
He added: "The Iraqi people will be looking to them to form an effective, responsible government that responds to their needs."
The Shias are expected to form a coalition government.
However, one Sunni party which won 11 parliamentary seats called on other parties to unite to form a parliamentary opposition.
Saadoun Zubaid of the Front for National Dialogue, told the BBC that an effective opposition was needed before democracy could be established.
Some Sunni politicians maintain the poll was fraudulent. International observers have also cited irregularities
Fareed Sabri, a spokesman for the main Sunni party, the Iraqi Islamic Party, said: "Our monitors on the ground in Baghdad counted that we had at least half of the seats in Baghdad.
"When the result went to the headquarters of the independent electoral committee they declared we had probably no more than 20% in Baghdad - this is not true."
Security measures were tightened in Baghdad and three Sunni Arab-dominated provinces on Friday amid fears of increased insurgent violence as the results were announced.
A roadside bomb attack, apparently targeting a US convoy, killed at least two civilians and injured several others in the city centre.
On Thursday, 15 people were killed in a double bombing in Baghdad.
Talk of unity
The final result of the 15 December poll was announced on Friday..
IRAQ ELECTION FACTS
The UIA's 128 seats was down from its total of 146 in the old transitional parliament elected last January.
The main Kurdish alliance also lost ground, down from 75 to 53, as a smaller rival group gained five seats.
Sunni Arabs increased their representation - a boycott of the January 2005 elections left them with just 17 seats in the old chamber.
The BBC's Jonny Dymond in Baghdad says there is much talk of a coalition of national unity comprising Shia, Sunni Arab and Kurdish blocs, but negotiations could take weeks or even months.
It is hoped that such a coalition will draw the sting from the insurgency, he says.
But one senior Shia politician has already expressed his doubts about that, and Iraq has, over the past three years, seen many false dawns.
Some 2,000 complaints of fraud, violence and intimidation have been noted since the election.
Allegations levelled at the conduct of the election include:
The observers did not reach any final judgement on whether the poll was free and fair.
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