Taliban fighters have launched a new assault on the capital of the northern Afghan province of Kunduz.
The attack comes a day after a bomb attack on a sports match in eastern Afghanistan killed nine people.
The Taliban stormed Kunduz from several directions early on September 28, with artillery and gunfire being heard in the city center, starting just after daybreak.
"Heavy fighting is ongoing in Khanabad, Chardara, and at Imam Sahed, the main entrances to the city," Kunduz police spokesman Sayed Sarwar Hussaini said. "We have enough forces and will drive them out soon."
Hussaini also said 20 Taliban fighters were killed and three Afghan police officers were wounded in the clashes.
An unidentified police spokesman said Afghan military helicopters were firing rockets at militants in Kunduz city's outskirts.
By mid-morning, the Reuters news agency reported that the Taliban fighters were inside Kunduz city limits and said buildings were on fire in the south of the town.
A spokesman for the Taliban, Zabihullah Mujahid, advised residents to remain indoors and said the militants had seized control of a 200-bed hospital in a district in the south of the city.
An unidentified police official confirmed that a government-run hospital had fallen into insurgent hands.
The head of the Kunduz provincial council, Mohammad Yusouf Ayubi, said the Taliban "launched their attacks from different directions using their full power."
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 sought to gain territory after NATO ended its combat mission in Afghanistan last December and pulled out the bulk of its troops.
Kunduz Province contains major roads that connect central and northern Afghanistan, including a road to the capital, Kabul.
On September 27, a suicide attack at a game of cricket in the eastern province of Paktika killed at least nine people and wounded more than 30.
There was no immediate claim of responsibility.
The explosion was caused by a motorcycle bomb, local officials said, and probably targeted local government officials watching the match.
A similar attack last year at a volleyball match killed at least 50 people in the same province.
Earlier on September 27, Afghan officials said Islamic State (IS) militants attacked checkpoints in Nangarhar Province, killing three police officers.
Provincial government spokesman Ahmad Zia Abdulzai said another eight policemen were wounded in the attack in the Achin district, bordering Pakistan.
He also said Afghan air strikes launched after the early morning attack killed 60 IS militants.
A UN report published on September 25 warned that IS was making inroads in Afghanistan, winning over a growing number of sympathizers and recruiting followers in 25 of the country's 34 provinces.