Two Taliban suicide bombers have killed up to 40 police cadets and civilians and wounded another 40 in an attack on a military convoy outside Kabul.

The bombers targeted buses carrying recently graduated cadets returning from a ceremony on the western outskirts of the city.

A police official who declined to be identified said three buses were attacked as they approached the Afghan capital from neighbouring Wardak province.

"Initial information we have is that two suicide bombers were involved and there are many casualties," he said.

An interior ministry official said at least 30 people were killed and 40 wounded.

The Taliban's official spokesperson Abdulqahar Balkhi has claimed responsibility on Twitter for the attack.

This is the second major terror attack in Kabul this month. It comes 10 days after an attack on a bus carrying Nepali security guards working for the Canadian embassy in Kabul that killed 14 people.

In April, at least 64 people were killed by a Taliban attack on a security services facility in Kabul in the deadliest bombing of its kind in Afghanistan since 2011.

Additional reporting by Reuters.