Afghanistan's defence ministry Wednesday shrugged off allegations that insiders were involved in a catastrophic insurgent attack on the country's largest military hospital, as it faces scathing public criticism over the assault.

Despite an earlier statement that the attack was inside job, the ministry says no evidence have been found to substantiate claims.

Read more: Insiders aided Kabul hospital attack, survivors claim

Gunmen disguised as doctors stormed Sardar Daud Khan hospital in Kabul last Wednesday, with multiple surviving staff telling AFP that insiders including two interns were among the attackers.

The carnage inside the heavily guarded hospital points at a spectacular intelligence failure and spotlights how insurgents have managed to infiltrate top government and military institutions in Afghanistan.

"We have no evidence it was an insider job," Deputy Defence Minister Helaludin Helal, head of a government investigation team, told reporters.

"The attackers entered the hospital using a Corolla car. One blew himself up and four others entered the building."

But his comment contradicts an earlier statement from his own ministry's spokesman Mohammad Radmanish, who conceded that "the attack was carried out from both outside and inside".

"This could not have been possible without the help of people inside," Radmanish said on Sunday.

Public anger has grown over the episode, with speculation swirling on social media that such a brazen attack on the tightly guarded hospital could not have happened without the complicity of high-ranking officials.

"The government is trying to cover up its embarrassing security failures and pretend there was no help from within," one Afghan said on Facebook Wednesday, echoing a deluge of similar comments.

Also read: Afghan security leadership under fire over hospital attack

The Afghan security leadership has also come under bitter criticism from lawmakers.

MPs are calling for the impeachment of Afghanistan's defence and interior ministers and the intelligence chief, as the country braces for an intense fighting season in the spring.

For his part, Helal said 24 hospital and military officials accused of "negligence of duty" are being prosecuted.

He added 50 people were killed by five attackers. But security sources and the survivors, some of whom counted dead bodies, previously told AFP that the death toll exceeded 100.

The savagery of the assault was characterised by how the assailants stabbed bed-ridden patients, threw grenades into crowded wards and shot people from point-blank range.

The militant Islamic State group claimed it was behind the attack via its propaganda agency Amaq ─ hours after the Taliban denied responsibility.

But the survivors AFP spoke to said some of the attackers "were in phone contact with handlers whom they addressed as Mullah Sahib", and attacked all but two wards on the hospital's first floor where Taliban patients were admitted.