An attacker seriously injured at least four people at a fitness center in the old town of the western German city of Duisburg on Tuesday evening, police said on Twitter.

Authorities confirmed that an operation led by police from the nearby city of Essen was under way and advised citizens to avoid the area.

Police searching for suspect

"There is still no arrest," a spokesman for the Essen police told dpa news agency.

A "stabbing or cutting weapon" was used in the attack, the police said. The injured were taken to a hospital, it added.

According to information from Germany's Bild newspaper, police suspect the perpetrator went on a rampage and is still on the run. The motive behind the attack is still unclear.

Police tweeted that "three of the victims have life-threatening injuries" with one other victim also suffering serious harm from the attack.

DW correspondent Joscha Weber, who is on the ground in Duisberg, said a police spokesman told him that they are searching for "at least one suspect." The SEK tactical police unit is involved in the search operation.

Weber said the streets were "calm" as the city center was blocked off. He noted that the police were searching even beyond the Duisburg city limits to find the perpetrator.

The police set up a media contact point for the press in front of the city hall in Duisburg. Witnesses who were at the crime scene are now being questioned.