An explosion inside the main military hospital in the Afghan capital Kabul on Saturday killed at least three people and wounded a number of others, the Afghan defence ministry told AFP.

"Three people have been martyred and a number of others have been wounded," the ministry press office said adding that the casualties included civilians and military personnel.

Afghanistan's Tolo television station said the blast was caused by a suicide attack and that a second attacker was still in the building. Tolo reported the blast took place as medical students were having lunch.

But the ministry said the cause of the blast was not immediately clear and that they were investigating.

The roads to the hospital were blocked and Afghan security forces including the Afghan spy agency's quick reaction forces cordoned off the hospital, while police and army were also on the scene.

There has been a recent spike in suicide bombings -- a key Taliban tactic -- and other attacks in Afghanistan.

Two rockets were fired overnight into Kabul, landing in an empty plot and causing no damage or casualties, interior ministry spokesman Zemarai Bashary told AFP.

Afghan forces are set to take increasing responsibility for security as foreign troops pull back. The worst fighting in Afghanistan usually comes in spring and summer and the fighting season is starting to get under way.

The Taliban announced the start of its spring fighting season at the end of last month.

On Thursday, 36 people were killed when they attacked a road construction company's compound in eastern Afghanistan in the biggest attack since they announced the start of the fighting season.

There are around 130,000 international troops in Afghanistan fighting the Taliban and other insurgents.

The conflict started with the US-led invasion of Afghanistan in 2001 following the September 11 attacks and has now run for nearly ten years.