An unknown assailant was shot dead as he attacked a guard outside the Iranian ambassador’s residence in Vienna, Austria, local police have confirmed.

“An unknown attacker armed with a knife has been shot dead by a guard from the Federal Armed Forces,” a police tweet said. The assault happened round midnight local time on Sunday. The guard was also injured in the incident and is now in hospital. The motive of the attack is not clear yet, law enforcement added.

#Aktuell: #Wenzgasse- Iranische Residenz: Unbekannter Angreifer mit einem Messer von Überwachungsposten des österreichischen Bundesheeres erschossen. Näheres derzeit unklar. — POLIZEI WIEN (@LPDWien) March 11, 2018

The perpetrator is a 26-year-old Austrian citizen, local police said in statement.

The 23-year-old guard was stabbed several times, but only suffered minor injuries thanks to a stab-proof vest. He initially used pepper spray to tackle the attacker. Later the two engaged in a tussle on the ground, during which the officer reached for his service weapon and fired several shots.

Authorities ordered heightened security measures at diplomatic missions in Vienna, according to local media. It is not clear if the ambassador was inside the building at the time of the attack.

Last week, four people, including a teenage girl, were seriously injured in two separate attacks in the Austrian capital. The perpetrator in one of the incidents was described as “uninhibited and psychotic,” according to witnesses.