Police prepare a tent near the Iranian ambassador's residence after an attack in Vienna, Austria, Monday, March 12, 2018. (AP Photo/Ronald Zak)

Asharq Al-Awsat

A 26-year-old Austrian was shot dead outside the Iranian ambassador's residence in Vienna on Sunday night after he attacked a guard with a knife, police said, prompting the authorities to order heightened security measures at diplomatic missions in the city.

The assailant "died on the spot" after the soldier opened fire, police spokesman Harald Soeros told AFP, adding that the attack took place just before midnight local time.

The motive for the attack is not yet known, police said.

The soldier, who was standing guard outside the building, suffered a wound to his upper arm in the attack. He was only saved by a stab-proof vest, according to the spokesman.

"He initially used pepper spray" against the assailant, Soeros said, before opening fire. He was believed to have fired his weapon at least four times.

According to early indications he "acted within the rules", said a spokesman from the ministry of defense quoted by the APA agency.

Police have ordered reinforcements around all diplomatic missions in Vienna, which is also home to many international organizations.

The incident comes just days after two knife attacks in Vienna, including one that left a couple and their 17-year-old daughter with severe injuries.

A 23-year-old Afghan national confessed to those assaults but denied any political motive.