US intelligence experts believe the Taliban has retaken control of about 10% of Afghanistan.
National Intelligence Director Mike McConnell told a US Senate committee that most of the country was under tribal control.
He said the government of President Hamid Karzai controlled only 30% of the national territory.
Another intelligence official, Defence Intelligence Agency director General Michael Maples, told senators the gains could be attributed in part to aid from al-Qaeda.
Afghanistan has in the last year seen its most violent period since the fall of the Taliban in 2001, increasing pressure on the US and its allies in NATO to beef up their military contingents.
At the same time, Gen Maples claimed al-Qaeda presented an increased threat to Pakistan, where he said it continues to plan, support and direct transnational attacks from its de facto safe-haven in the frontier provinces.
