At least 38 people have been killed and some 70 others injured in an attack by heavily-armed militants on Afghanistan's largest military hospital in the capital, Kabul.

Afghan Defense Ministry spokesman Dawlat Waziri said on Wednesday that four attackers were also killed during the assault on the hospital, which is located in a diplomatic area, home to several heavily-guarded embassies, including the US embassy.

"Most of the victims are patients, doctors and nurses," media outlets quoted Waziri as saying.

Initial reports had put the death toll at three. Officials revised the figure after security forces checked the buildings in the hospital complex.

Media reports earlier said gunmen entered the hospital after a huge explosion and gunfire. Local TV footage showed some of the victims trapped on the ledge of a top-floor window.

Medical staff who hunkered down in the hospital wards posted desperate messages for help on social media websites.

"Attackers are inside the hospital. Pray for us," a hospital staff member wrote on Facebook.

Hospital administrators said gunmen wearing white laboratory coats began spraying bullets after a bomber blew himself up at the backdoor entrance.

Abdul Qadeer, a nurse, said, "I saw one of the attackers, armed with an AK-47 and dressed as a doctor, shooting at patients and guards on the third floor."

"They shot my friend but I managed to flee... I had to jump over the barbed wire to escape."

The Takfiri Daesh terrorist group has claimed responsibility for the deadly six-hour attack, which sparked chaos inside the 400-bed facility.

Iran slams terror attack

Iran immediately condemned the inhuman and evil attack in Kabul and expressed sympathy with the Afghan government and nation as well as the families of the victims.

"The only way to combat the scourge of terrorism in Afghanistan and the region is global resolve and consensus and blocking the paths of spiritual and material aid and support for these terrorist groups," Iran's Foreign Ministry Spokesman Bahram Qassemi said on Wednesday. 

Kabul has seen a wave of terrorist attacks on government buildings over the past days. Last week, at least one person was killed and 35 were wounded in attacks on a police precinct in western Kabul.

The rise in attacks comes as the Taliban is reportedly preparing for its annual spring offensive in the war-torn country.

Taliban has warned the Daesh ringleader, Ibrahim al-Samarrai also known as Abu Bakr al-Baghdadi, against "waging a parallel insurgency in Afghanistan."

The militant group has urged the Daesh leader to keep his men out of Afghanistan and stop supporting those elements that are recruiting young militants in Taliban strongholds.