Taliban fighters launched a three-pronged offensive on the capital of the northern Afghan province of Kunduz on Monday, fighting their way through the main entrances to the city, burning buildings and briefly taking over a hospital.Breaching a provincial capital marks a troubling milestone in the nearly 14-year-old insurgency, though Afghan forces this year have driven the Taliban from most territory they've gained in the warm-weather fighting season.The assault was the second time this year that the Taliban have besieged Kunduz city, as the NATO-trained Afghan police and army fight largely without the help of foreign forces.By mid-morning, the Taliban fighters were inside the city limits. A Reuters witness saw buildings on fire in the south of the city, and he saw Taliban fighters entering a 200-bed government-run hospital.Dozens of panicked residents fled to the city's main airport but were turned away by security forces. By afternoon, the fighting had reached about a kilometer (0.62 mile) from the city's main government compound, according to a Reuters witness.Afghan military helicopters were firing rockets at militants in three areas on the city's outskirts, a police spokesman said. Artillery and gunfire could be heard in the city center from just after daybreak."Right now heavy fighting is ongoing in Khanabad, Chardara and at Imam Saheb, the main entrances to the city," Sayed Sarwar Hussaini, a spokesman for Kunduz police, said. "We have enough forces and will drive them out soon."He said 20 Taliban fighters were killed and three Afghan police wounded in the clashes.But if Afghan forces cannot drive out the Taliban from any of the city's three main entrances, it would appear be difficult for the government to maintain control.Special forces of the Afghan police and army were on their way from neighboring Balkh province to help defend Kunduz, a Balkh police commander said.