KABUL (AFP) - At least 20 Afghan cadets were killed when a suicide attacker blew himself up at the entrance of Kabul Police Academy on Friday, officials said.

"The attacker was wearing police uniform... when he detonated his explosives, 20 cadets were killed and 20 more were wounded," a senior Afghan intelligence official told AFP, requesting anonymity.

The bomber managed to place himself in a queue as police trainees were waiting to be searched before entering the academy, the official said.

The Taleban were behind the suicide attack, the spokesman for the group, Zabihullah Mujahid, told AFP.

Another police official confirmed that toll while a third senior security source told AFP that 25 cadets were killed in the attack.

The incident, which comes as cadets were returning to the academy after their two-day weekend, marks a serious breach of security at a premier training institute for Afghan security forces.

Heavily-armed security officials cordoned off the area and ambulances with wailing sirens were seen rushing to the scene.

The academy in west Kabul is a premier training institution for police forces in Afghanistan, with between 2,000 and 3,000 cadets graduating every year.

The suicide bombing comes less than 24 hours after a truck bomb tore through central Kabul, killing 15 civilians and wounding 240 others in the first major attack in the Afghan capital since the announcement of Taleban leader Mullah Omar's death.