The blast took place in Paghman district in western Kabul as the convoy was returning from an overnight operation, interior ministry spokesman Najib Danish told AFP.

"It is still not clear whether the attacker was on foot or driving a vehicle," the spokesman added.

Another security official requesting anonymity said the assailant had used a car bomb to target the convoy.

Attacks on Afghan forces by the Taliban and the Islamic State group have been inflicting record-high casualties on security personnel this year.

Afghan security forces, beset by killings and desertions, have been struggling to beat back insurgents since US-led NATO mostly left them on their own three years ago.

In November President Ashraf Ghani said nearly 30,000 Afghan soldiers and police officers have been killed since 2015 -- a figure far higher than anything previously acknowledged.

Earlier this month, Lieutenant General Kenneth McKenzie -- who has been nominated to lead the US military's Central Command -- said the death rate among Afghan forces will no longer be sustainable unless urgent measures are taken to address recruiting and training issues.

The early morning attack in Kabul came just hours after an overnight assault on a checkpoint in Arghistan district of southern Kandahar province by Taliban fighters killed at least eight Afghan police officers according to the provincial media office..