At least three U.S. airstrikes had targeted Taliban fighters near the city by early Wednesday. U.S. Army spokesman, Col. Brian Tribus, said that U.S. and NATO coalition advisers, including special forces, were also at the scene "in the Kunduz area, advising Afghan security forces."

He denied reports that they were fighting on the ground, stressing the forces were there in a non-combat, train-and-assist role. "But these are dangerous situations and if they need to defend themselves, they will," Tribus added.

Residents reported increasing distress as insurgents put the city under virtual lockdown, blocking roads and setting up checkpoints to prevent people from leaving. The roadblocks also kept essential supplies of food and medicine from reaching the city of some 300,000 people.

Afghan military planners, backed by U.S. and NATO advisers, were bringing in units from across the nation's security forces — army, air force, police, special forces and commandos. Coordination and planning was likely to take some time: The Afghan forces have been fighting alone only since the withdrawal last year of U.S. and NATO combat troops, and the learning curve has been steep and casualties high.

The protracted crisis is a huge blow for President Ashraf Ghani, who took office a year ago pledging to restore peace to Afghanistan, and who had promised Kunduz would soon be back under government control.

But the complicated nature of the fight and the need for a cross-pillar approach under a single command means it could be some time before progress is visible, military experts said.

"Neither heavy artillery nor airstrikes on the city itself are practical as the Taliban have embedded themselves within the civilian population," said retired Afghan army Gen. Atiqullah Amerkhail.

When the strategy is rolled out it is likely to see Afghan special forces airdropped to clear major roads and narrow alleyways, with follow-up reinforcements sent in to hold the cleared areas, he said.

Tribus, the U.S. and NATO mission spokesman, said Afghan forces, who have faced sharp criticism and suffered from low morale as the Taliban spreads its fight across the country, have made improvements.

"Logistics are vital to operations like that in Kunduz and they have made improvements in planning operational logistics," he said.

Retaking Kunduz will be "another test" for Afghanistan's armed forces, Tribus said.

Taliban fighters were also gearing up for a long fight and were seen planting bombs and mining roads in and out of the city.

The situation in Kunduz has heightened concerns among U.S. military commanders that current plans to cut the number of American troops in Afghanistan may be premature. In what would be a major policy shift, the military leadership wants to keep at least a few thousand U.S. troops in Afghanistan beyond 2016, citing the fragile security situation.

About 9,800 U.S. troops are in Afghanistan, and the top U.S. commander in Afghanistan, Army Gen. John F. Campbell, has given the administration several options for gradually reducing that number over the next 15 months, U.S. officials said. All call for keeping a higher-than-planned U.S. troop presence, said the officials, who spoke on condition of anonymity because they weren't authorized to speak about plans still under consideration.

The timing of a decision on new U.S. troop levels is unclear. Campbell is scheduled to testify to Congress next week, including on the effectiveness of Afghan forces after a tough summer of fighting.

The U.S. and NATO troops play a vital role in training and assisting the Afghan forces. U.S. airstrikes Tuesday and Wednesday hit Taliban positions during fierce fighting for control of Kunduz's airport, a few miles outside the city, before the Taliban retreated under fire, according to residents. The airport remained in Afghan government hands.

Another strategic site did not. A lawmaker from Kunduz, Malim Chari, told The Associated Press that the ancient fortress of Bala Hissar, which the Afghan military had used as a security post, fell early Wednesday afternoon.