At least 20 people have been killed in fighting between security forces and Islamic State gunmen raiding an Afghan jail, officials said Monday, after the jihadist group claimed the ongoing attack.

Fighting was raging one day after gunmen stormed the prison in the eastern city of Jalalabad. Nangarhar provincial hospital spokesman Zaher Adel told AFP that 20 people had been killed so far -- including security personnel -- while the governor's spokesman put the toll at 21.

The toll was expected to rise, Adel warned, as many of the more than 40 wounded people were in serious condition. The raid shattered the relative calm of a ceasefire between the Taliban and Afghan government forces that started Friday to mark the Muslim holiday of Eid al-Adha.

The three-day truce ended Sunday, with Kabul hoping it could be extended. The IS news outlet, Amaq, announced that its fighters were behind the prison raid, according to the SITE Intelligence Group.

About 700 prisoners who escaped during the raid have been re-arrested, the Nangarhar governor's spokesman Attaullah Khogyani told AFP. Fighting was ongoing as gunmen remained holed up inside and outside the prison, Khogyani said.

Several armoured vehicles and scores of security personnel were deployed in the area, and gunfire and explosions could be heard. At the time of Sunday's attack, more than 1,700 prisoners were in the prison, most of them fighters of IS and the Taliban, a security source told AFP.