Six years after being ousted in the US-led invasion, the Taleban have retaken about 10% of Afghanistan, US intelligence chief Mike McConnell says.
Afghan President Hamid Karzai's government was said to lack control
The government controlled just 30% of the country, and the rest was under tribal control, the director of national intelligence told senators.
But that assertion has been denied by the Afghan government as incorrect.
The Committee on Armed Services heard the Taleban's resurgence was due partly to increased support from al-Qaeda.
The last 12 months have seen the worst violence in Afghanistan since 2001.
The head of the US Defence Intelligence Agency told the same committee major problems remained in trying to crack down on the lawless tribal area on Afghanistan's border with Pakistan.
Lt Gen Michael Maples said the Pakistani military was not trained for that fight.
He added that it would take three to five years to address those deficiencies and to make a difference.
"Pakistani military operations in the [region] have not fundamentally damaged al-Qaeda's position," Gen Maples said.
"The tribal areas remain largely ungovernable and, as such, they will continue to provide vital sanctuary to al-Qaeda, the Taleban and regional extremism more broadly."
In Washington, independent foreign policy experts have previously warned that Afghanistan largely remains a failing state.
The latest assessment from US military chiefs underlines the cause for concern facing Afghan President Hamid Karzai's government, says the BBC's Jonathan Beale in Washington.
It has strongly rejected the claim that it controls only 30% of the country.
"This is far from the facts and we completely deny it," the Afghan Defence Ministry said in a statement.
"All Afghan people know that in the 34 provinces of Afghanistan and in more than 360 districts... the government has control," the statement said.
Ministers argue that just because tribal leaders are encouraged to ensure security in some areas, that does not mean that the government is not present there.