The assessment echoes a finding by the director of U.S. national intelligence, who told a Senate committee last month that Taliban insurgents control about 10 percent of the country.
That judgment by Michael McConnell was hotly disputed by Afghan officials. Afghanistan's intelligence chief, Amrullah Saleh, said in response earlier this month that only eight of Afghanistan's 364 districts were not in government control.
But the U.N. report, released in New York on Monday, said 36 districts including most of those in the east, southeast and south are largely inaccessible to Afghan officials and aid workers.
"Despite tactical successes by national and international military forces, the anti-government elements are far from defeated," the report said.
The report said violence last year was at the highest level since a U.S.-led offensive toppled the hard-line Taliban regime in late 2001. There were 160 suicide attacks and 68 thwarted attempts in 2007, compared to 123 suicide attacks and 17 failed attempts in 2006, it said.
Afghanistan had more than 8,000 conflict-related deaths last year, including 1,500 civilian deaths, the U.N. said.
In the latest violence, police clashed with Taliban fighters in the Dihrawud district of southern Uruzgan province Monday, leaving 10 militants dead and two officers wounded, the provincial police chief, Gen. Juma Gul Himat, said.
In western Afghanistan, Afghan police backed by NATO-led troops killed four suspected criminals following a spate of kidnappings and robberies, said Rauf Ahmadi, a regional police spokesman in Herat.
The operation in the Guzara area of Herat also captured 15 other people suspected of involvement in criminal activities, he said.
The police raid came after the kidnapping last week of a doctor's son. All medical workers in Herat city have been on strike the last four days, demanding that the government do more to provide security in the province, officials said.
Occasional kidnappings of foreigners in Afghanistan receive wide publicity, but Afghans are kidnapped for ransom much more often.