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The attack continued throughout the night and Monday morning
ISIS said it was behind a major attack on a central prison in the eastern city of Jalalabad, which led to hundreds of prisoners trying to escape.
The attack began on Sunday evening, when gunmen detonated car bombs at the prison entrance.
It continued overnight and Monday morning, as ISIS militants were fighting the security forces.
A Nangarhar Province spokesman said at least 21 people were killed.
According to Atallah Khoujiani, 43 people were injured and taken to a regional hospital. He said three attackers were killed, but others were still resisting security forces from the top floor of an apartment building.
A security source told Agence France Presse that the prison includes more than 1700 prisoners, most of them are Taliban and ISIS militants. It was not immediately clear if the attack had released certain prisoners from the compound.
The attack occurred on the third and last day of a temporary ceasefire between the Afghan government and the Taliban, and hundreds of Taliban prisoners were released in an attempt to trigger peace talks between the two sides.
The Taliban – a powerful rival to ISIS – had said earlier that they were not responsible for the attack.
Sunday's attack came one day after the Afghan intelligence agency announced that the country had killed Asadullah Orakzai, a senior ISIS commander, near Jalalabad. Orakzai was allegedly involved in several fatal attacks against Afghan security forces.
Nangarhar Province was the first ISIS stronghold in Afghanistan. It still has a foothold there despite government officials claiming last year that the group's local branch – known as the Islamic State of Khorasan – was completely defeated in the province.
Nangarhar has already seen fatal attacks this year, including a suicide bombing on May 12 that killed 32 mourners at the funeral of a police chief.