VIENNA, March 12 (Xinhua) -- A knife attacker who ambushed a guard was shot dead near the residence of Iranian ambassador in Vienna, Austria late Sunday, police said.

The assailant, an Austrian citizen aged 26, ambushed a lone guard on sentry duty outside the residence before midnight, and injured the guard in the arm.

The assailant was later shot by the guard and died on the spot, according to police.

The attacker's motive remains unknown, police said.