The "Taliban had a good year last year, they are trying to have a good one this year," Mattis told the Senate Armed Services Committee.
"Right now I believe the enemy is surging."
Lawmakers repeatedly asked Mattis for an update on Afghanistan and sought reassurance that America would be able to effectively help Afghan partners reverse a stalemate against the resurgent Taliban.
Mattis said he expected to present a revised strategy by mid-July. He is believed to be mulling sending an additional 3,000 to 5,000 US troops.
"We are not winning in Afghanistan right now. We will correct this as soon as possible," he said.
The Afghan conflict is the longest in American history, with US-led NATO troops at war there since 2001, after the ousting of the Taliban regime.
The ongoing security crisis has seen not just the Taliban gain ground, but also the emergence of other groups such as a local Islamic State affiliate.
On Saturday, the Taliban claimed responsibility for an insider attack in which an Afghan commando turned his weapon on American troops, killing three and wounding another.
US troops in Afghanistan number about 8,400 today, and there are another 5,000 from NATO allies, who mainly serve in a training and advisory capacity.
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