Assistant Secretary of State for Central and South Asian Affairs Richard Boucher said efforts to extend the rule of President Hamid Karzai's government deeper into the provinces had run into tougher-than-expected resistance.
"As this extension of government goes out, we are challenging those people and they are challenging us back," he told reporters at the heavily fortified US embassy in Kabul.
"I think we have all been surprised by the intensity of the violence this year. It has a number of factors: part of it is drug money linking up with the insurgency. Part of it these people have the ability to operate in and out of Pakistan.
"But we need to deal with it."
PAKISTAN ACCUSED OF BACKING TALIBAN
Some senior Afghan officials accuse Pakistan of continuing to back the Taliban, its former protege, with money, training and other assistance, a charge Islamabad denies.
Afghan intelligence officials say they have presented solid evidence to the United States. However Washington says there is no official Pakistani support for the Taliban, but that more needs to be done to stop the group and other militants and criminal gangs moving freely across the rugged border.
Boucher said Pakistan was using military, economic and other measures to prevent the Taliban using its territory as "a place of refuge or of support".
Boucher's visit came as NATO forces killed up to 22 Taliban in an air strike in Kandahar province.
The clash happened on Wednesday in an area near the provincial capital that was the scene of a major two-week NATO offensive in September that killed hundreds of Taliban.
District police chief Ghulam Rasool said at least six more guerrillas were killed in another battle in the area on Thursday.
And local authorities said five Taliban and two policemen were killed in a gunbattle in Nimruz province, which borders Iran, on Thursday.
The NATO-led International Security Assistance Force completed its take-over of security nation-wide from a US-led coalition in October. As part of that, it has pushed more troops into the volatile south of Afghanistan, including Kandahar.
NATO and US commanders say this has led to an upsurge in fighting this year.
But many analysts say the Taliban's resurgence has been fuelled by record opium crops and growing frustration and anger at a lack of reconstruction, development and jobs.

Copyright Reuters, 2006