At least 20 Afghan cadets were killed when a suicide attacker blew himself up near the 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.

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