Taliban fighters launched a new offensive on the capital of northern Afghan province of Kunduz from three directions on Monday, police said, as government forces fought to keep the hard-line Islamist insurgents from entering the city. The assault was the second time this year that the Taliban have threatened to take ground inside Kunduz city, as the NATO-trained Afghan police and army fight largely without the help of foreign forces. Breaching a provincial capital would be a 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. Afghan military helicopters were firing rockets at militants in three areas on Kunduz city's outskirts, a police spokesman said. Artillery and gunfire could be heard in the city center starting 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 were wounded in Monday's clashes. But if the Taliban defeat Afghan forces at any of the three main entrances, it would be difficult for the government to maintain control. Taliban spokesman Zabihullah Majahid urged Kunduz residents to stay indoors. "The Mujahideen (Taliban fighters) are trying to avoid any harm to Kunduz residents. Residents have to be assured they will not face any problem from our side," Mujahid said on his official Twitter account. The once-quiet north of Afghanistan has seen escalating violence in recent years. Kunduz city was the center of fierce fighting in April as the Taliban, driven from power by a 2001 U.S.-backed military intervention, sought to gain territory after last year's end of NATO's combat mission. A scaled-down NATO mission now mostly trains and advises Afghan forces, although U.S. drones still target militant leaders and an American counter-terrorist force also operates in the country.