Gen. David Petraeus said NATO forces had reversed some of the gains the Taliban had made in recent years in the southern provinces of Helmand and Kandahar and around Kabul.
But in an interview with NATO TV from Kabul, Petraeus said: "I would not say we have reversed the momentum in all areas by any means."
"In some we have reversed it, in some we have blunted it, in some perhaps the Taliban are still trying to expand," he said.
He said the insurgents were fighting to take back the market town of Marjah in Helmand province, which he described as "one of the most important command and control areas for the Taliban and the nexus for the illegal narcotics industry."
He also noted that NATO's campaign to secure the southern city of Kandahar had just begun.
"But clearly there's a lot more work to be done with the Taliban fighting back very hard," he said. "This is really (Taliban leader) Mullah Omar's hometown, this is the iconic place of the Taliban and it's very important for them and it will be tough."
He also noted that the Afghan security forces were growing faster than expected, with the army numbering 134,000 men and the police slightly more.
The NATO-led force has about 140,000 troops. Taliban guerrillas are estimated to number between 25,000 and 30,000.