Months of research across the country show how areas the Taliban threaten or control have surged since foreign combat troops left in 2014.
The Afghan government played down the report, saying it controls most areas.
But recent attacks claimed by Taliban and Islamic State militants have killed scores in Kabul and elsewhere.
Afghan officials and US President Donald Trump responded by ruling out any talks with the Taliban. Last year Mr Trump announced the US military would stay in the country indefinitely.