US Defence Secretary Robert Gates has said he is disappointed by the lack of political progress in Iraq.
A security surge in Baghdad was meant to allow for reconciliation
The Bush administration had probably underestimated the depth of mistrust among Iraq's various factions, he said at the end of a Middle East tour.
Recent developments were discouraging, he said, citing the withdrawal of the main Sunni Arab bloc from government.
A recent surge in US troop numbers was designed to provide a breathing space to pursue reconciliation, he added.
Mr Gates's comments have been seen by many as a vote of no confidence in the abilities of the Iraqi Prime Minister, Nouri Maliki.
Speaking to reporters on his way back from the Middle East, Mr Gates admitted political reconciliation between Iraq's factions had been slower than expected.
Last week, the Sunni Iraqi Accordance Front, which had six cabinet ministers, pulled out of the government saying the Shia-led administration had failed to meet a list of demands, including one urging tough action against Shia militias.
"In some ways we probably all underestimated the depth of the mistrust and how difficult it would be for these guys to come together on legislation," Mr Gates said.
"The kinds of legislation they're talking about will establish the framework of Iraq for the future so it's almost like our constitutional convention...
Despite Mr Gates's assessment of the political situation in Iraq, the US military believes developments on the security side have been more encouraging.
Civilian casualties in Iraq have dropped by almost a third since the surge began seven weeks ago and there have been reduced levels of violence in some areas of Baghdad.
Democrats have called for a withdrawal of US troops from Iraq
US officers have also said growing opposition among Iraqis to extremist groups such as al-Qaeda in Iraq has resulted in closer co-operation with American forces and a real chance of progress.
But the BBC's defence correspondent, Rob Watson, says that although US commanders are pleading for more time, the clock is clearly ticking in Washington.
Congress is impatiently awaiting a progress report from the US government in September, though many have already decided the surge will not work, our correspondent adds.
The latest opinion polls also suggest that 60% of Americans want to see a troop withdrawal.
On Friday, the US military announced the combat deaths of four more soldiers in Baghdad, including three killed on Thursday in a roadside bombing.
An aide to Iraq's top Shia cleric, Ayatollah Ali Sistani, was killed in Najaf, security officials said.
In Kirkuk, relatives mourned the deaths of five brothers who were kidnapped and then killed after their family was unable to pay a ransom.
Police found a sixth brother, aged eight, weeping but unharmed next to the bodies, all of which had gunshot wounds to the head.
Correspondents say the presence of the small boy at the scene of the murders has touched a nerve in Iraq, although the country witnesses many such murders every day.