JALALABAD (AFP) - Afghan authorities were searching on Tuesday (Aug 4) for about 270 inmates - most of them fighters of Islamic State in Iraq and Syria (ISIS) - who remained on the loose after escaping during a deadly prison raid.

At least 29 people were killed when ISIS gunmen attacked the facility in Jalalabad on Sunday, with fierce fighting lasting until Monday afternoon.

More than 1,300 inmates tried to escape, a senior Afghan security official told AFP on condition of anonymity, but most were either swiftly re-arrested or surrendered when surrounded by security forces.

But some 270 prisoners are "still on the loose", the official said.

"Most of those who escaped are from ISKP," he said, referring to ISIS' Afghan branch, known as the Islamic State in Khorasan Province.

They included militants responsible for several bloody attacks, a second security official told AFP.

Nangarhar provincial governor's spokesman Attaullah Khogyani confirmed many prisoners were still missing, but could not say how many were ISIS members.

Officials also said that security forces had shot dead on Tuesday another group of ISIS fighters who had launched a mortar attack targeting Jalalabad airport from a neighbourhood in the city around the same time when the prison raid commenced.

Several mortar shells and other ammunition seized from a house in the neighbourhood were shown to journalists.

The brazen prison raid came a day after Afghanistan's intelligence agency announced the killing of a top ISIS commander near Jalalabad.

Nangarhar province, where ISIS got its first foothold in Afghanistan, has seen repeated attacks by the group, including a suicide bomb that killed 32 mourners at a funeral in May.

CALM SHATTERED

The attacks have continued even though officials last year claimed ISIS had been defeated in Nangarhar.

"A large number of their leadership was arrested or killed... so it (the prison raid) was some sort of revenge to free some of their comrades," a senior Afghan security official said.

Some local officials and analysts have, however, cautioned that elements of group remained.

"They (ISIS) want to challenge the monopoly of the Taleban over militancy or anti-state violence in the country," said Abdul Sayed, independent researcher on militancy in Afghanistan and Pakistan.

He said the Taleban, Kabul and Washington miscalculated about ISIS' defeat in Afghanistan.

"They (ISIS) now seem to have changed their strategies to (launch) high profile terrorist attacks in Nanagarhar to Kabul to provide proof of their existence," Sayed told AFP.

The prison raid shattered the relative calm of the final day of a three-day ceasefire between the Taleban and Afghan forces.

ISIS was not part of the truce.

The Afghan government still attempted to link the raid to the Taleban, which it said had violated the ceasefire 38 times.

Acting Interior Minister Massoud Andrabi said on Twitter that ISIS's new head in Afghanistan, Shahab Al-Mahajir, was a member of the Haqqani network, considered linked to the Taleban.

Washington has branded the Haqqani network, accused of carrying out numerous deadly attacks in Afghanistan, a terrorist organisation.