Hundreds of Taliban fighters have seized parts of the strategicnorthern Afghan city of Kunduz, reports say.
There is fierce fighting after militants launched a surprise attack from several directions. There are casualties on both sides, police say.
Taliban fighters have taken government buildings in the city centre.
Monday's attack appears to be one of the most significant mounted on a provincial capital by the Taliban, correspondents say.
Government officials say that reinforcements have been sent to the city.
"The Taliban have taken over our neighbourhood... I can see their fighters all around," an AFP news agency journalist inside the city reported.
Kunduz province has seen a number of attacks since April, with the Taliban joining forces with other insurgents.
The city is symbolically significant to the Taliban as it was their former northern stronghold before their government was overthrown in 2001.
The assault comes a day after a bomb attack on a sports match in eastern Paktika province killed nine people.
'Fighter selfies'The Taliban said they had also entered a hospital in Kunduz, although local media reports say they only stayed briefly.
Pictures on social media appeared to show fighters taking selfies inside the hospital.
The government claims to have killed at least 20 militants.
Kunduz province contains major roads that connect central and northern Afghanistan. Correspondents say it also supplies half of Afghanistan's rice crop.
Militant violence has increased across Afghanistan since the departure of most US and Nato forces last year.
On Sunday, a bomb attack on a sports ground in Paktika left nine dead and dozens injured. No group has said it carried out that attack.
Also on Sunday, 300 fighters allied to the so-called Islamic State attacked checkpoints in Nangarhar province.
Two policemen were reported killed in the assault, while local officials said 60 militants were killed.