Students were gunned down "one by one" as they leapt from the windows of Kabul University classrooms when militants stormed the campus earlier this week, a survivor told AFP Wednesday.

At least 22 people, mostly students, were killed and another 27 wounded when three gunmen stormed the university on Monday, spraying classrooms with bullets in an assault claimed by the jihadist Islamic State group.

Mohammad Qasim Kohistani, 22, a student of public policy at Kabul University, one of Afghanistan's main educational institutions, described how the seven-hour attack unfolded.

"We were waiting for our teacher to come... suddenly, we heard loud sounds coming from the first floor," Kohistani, who was in a classroom on the second floor, told AFP. "We knew something was wrong, we started jumping from the windows."

Kohistani took refuge under a window ledge with his friends, a bloody scene unravelling above him as the attackers went room to room searching out victims.

"After a gunman took over our class he started shooting fleeing students from the window," he said. "The (other two) gunmen were shooting students one by one from a close range," he added.

Hundreds of students fled, some by scrambling over perimeter walls and onto the streets, others by barricading themselves inside rooms until they were rescued by Afghan special forces.

Best friend killed

Among those killed was Kohistani's best friend, Ahmad Ali, who stopped to help another friend, Roqia.

"He didn't jump from the window because Roqia had fainted," he explained. "He wanted to save her but the gunman shot both of them in the head and killed them."

Kohistani, who sprained a leg when he jumped, stayed hidden for hours on campus.

"It was the most horrible moment of my life," he said. "All through the day we were hearing their wails and cries for help but we were helpless."

After the attack ended, Kohistani came to learn that the attackers had killed everyone inside one of the classrooms below his. "I am still in trauma. Yesterday, I was not even able to talk," he said.

Officials said two attackers were shot dead by security forces, while the third blew himself up during the assault.

The attack was claimed by IS, the second such assault the group had carried out on an educational centre in less than two weeks in the capital.

However, Vice President Amrullah Saleh has blamed the Taliban and their allies in Pakistan for the attack. The Taliban have denied it was involved.

Afghan security forces have been grappling with surging violence that has only worsened in recent months despite the government holding peace talks with the Taliban in Qatar.

Afghanistan held a national day of mourning on Tuesday, while students demonstrated outside Kabul University, holding banners which read "Stop killing us".