At least 19 people were killed and 50 others injured in an attack on a military hospital in Kabul on Tuesday, the latest assault to rock Afghanistan since the Taliban seized power.

Two blasts struck close to the entrance of the sprawling site followed by reports of shooting inside the hospital, the Taliban and a witness said. 

"Nineteen dead bodies and about 50 wounded people have been taken to hospitals in Kabul," a health ministry official who asked not to be named told AFP.

The Taliban spent 20 years waging an insurgency against the ousted US-backed government. 

Now they face the struggle of bringing stability to Afghanistan, which has been hit in recent weeks by a series of bloody assaults by the Islamic State group's local chapter.

Tuesday's attack has not yet been claimed by any group.

"I am inside the hospital. I heard a big explosion coming from the first checkpoint. We were told to go to safe rooms. I also hear guns firing," a doctor at the Sardar Mohammad Daud Khan hospital in Kabul told AFP.

"I can still hear gun firing inside the hospital building. I think the attackers are going from room to room... like the first time it was attacked," the doctor added.

The hospital, which treats wounded soldiers from both the Taliban and former Afghan security forces, was previously attacked in 2017, when gunmen disguised as medical personnel killed at least 30 people in an hours-long siege. 

A Taliban media spokesman confirmed two explosions hit the hospital area on Tuesday. 

"One explosion has happened at the gate of the military hospital and a second somewhere near the hospital, this is our initial information, we will provide more details later," he told AFP.

Qari Saeed Khosty, a spokesman for the interior ministry, said Taliban special forces had rushed to the scene to secure the area after "a bomb" exploded. 

AFP journalists heard a second blast followed by gunfire.

- Ambulances speeding through Kabul -

Although both IS and the Taliban are hardline Sunni Islamist militants, they have differed on the minutiae of religion and strategy.

IS have claimed four mass casualty attacks since the Taliban takeover on August 15, including suicide bomb blasts targeting Shiite Muslim mosques. It regards Shiite Muslims as heretics. 

In the 2017 attack on the military hospital, militants went room to room killing people, switching to knives when they ran out of ammunition. 

That attack was claimed by the Islamic State group, and the Taliban denied responsibility.

However, survivors told AFP that the attackers chanted "Long live Taliban" in Pashto and attacked all but two wards on the hospital's first floor where Taliban patients were admitted.

Black smoke 

An Italian NGO which runs a separate hospital in the capital tweeted on Tuesday that it has received nine patients with injuries from the blast site.

Pictures shared on social media showed black smoke billowing into the air after the explosions, the first of which went off at around 1:00 pm (0830 GMT).

AFP journalists saw Taliban fighters racing to the scene in at two armoured personnel carriers (APCs) and pick-up trucks.

Roads close to the heavily fortified "Green Zone" where the buildings of several former Western embassies are located were closed off to traffic and Taliban guards scaled up searches.

Sirens could be heard in the streets and ambulances were seen speeding towards the blast site, and two helicopters circled above the city throughout the afternoon.