Taliban fighters mounted a coordinated assault on the northern city of Kunduz overnight, attacking from four directions and entering the city itself, a senior city police official said.
Sheer Ali Kamal, commander of the 808 Tandar police zone in Kunduz said the attack began at around midnight (1930 GMT Sunday) and fighting was still going on in and around the city.
"We are putting all our efforts together to push them back," he said.
Kunduz, which fell briefly to the Taliban a year ago, has seen repeated bouts of heavy fighting and was seriously threatened in April.
A Reuters reporter saw at least five Taliban fighters armed with AK-47 assault rifles, machines guns and rocket-propelled grenades in the city.
The attack come as the Taliban have stepped up operations in different parts of Afghanistan, including the strategic southern province of Helmand, where they have been threatening the provincial capital of Lashkar Gah.