Secretary-general says 4,000 more troops will be needed to safeguard August ballot.
Shir Mohammad Akhonzada, the former governor of Helmand, said that Khan fled the country during Taliban rule but later returned as Helmand’s intelligence chief.
Taliban fighters have maintained their armed struggle to regain power from the Western-backed administration.
No group has yet claimed responsibility for the attack. The government said the bomb was controlled remotely.