The Pentagon says people support President Karzai in 29 key districts
The US defence department has said that only a quarter of what it regards as key regions in Afghanistan support the government of President Hamid Karzai.
The Pentagon said in a report that much of the population was either neutral to the central Afghan authorities or supportive of the Taliban insurgency.
It blamed government corruption and lack of efficiency as major reasons for people's distrust of the authorities.
Afghanistan has seen a sharp increase in violence in the past year.
The Pentagon said that despite this, opinion polls suggested Afghan people believed their security was improving.
In its 152-page report, released ahead of President Karzai's upcoming visit to Washington, the Pentagon said: "While Afghanistan has achieved some progress on anti-corruption... real change remains elusive and political will, in particular, remains doubtful."
It said people support President Karzai's government in only 29 of the 121 Afghan districts considered most strategically important in the war effort.
President Karzai won the 2009 presidential election, which was criticised for widespread fraud.
Separately, the report said that Taliban militants were coming under "unprecedented pressure".
"From the insurgents' perspective, this strain has been compounded by the recent high-profile arrests of several Pakistan-based insurgent leaders by the Pakistani authorities and the removal of many Afghanistan-based commanders," it said.
Correspondents say that the reputation of President Karzai - once a darling of the international powers - has plummeted following repeated accusations from the US and other nations that he has allowed unchecked corruption.
The report said that popular anger at his government, which is widely seen as corrupt and inefficient, has allowed the Taliban to "perceive 2009 as their most successful year".
"Expanded violence is viewed as an insurgent victory, and insurgents perceive low voter turnout and reports of fraud during the past presidential election (in August 2009) as further signs of their success," it said.
Pentagon figures show that "violence is sharply above the seasonal average for the previous year - an 87% increase from February 2009 to March 2010".
"Although the overall security situation has stabilised somewhat since the end of 2009, violence during the current reporting period is still double that for the same period in 2008-2009," the report said.