Police in Germany's western state of North-Rhine Westphalia arrested three more people on Sunday over an alleged plot to attack the historic cathedral in Cologne. One person was previously detained on Christmas Eve.

Cologne police chief Johannes Hermanns told a news conference that they believed the perpetrators planned to strike on New Year's Eve and that the "alleged means of attack" was a vehicle.

The arrests were made in the cities of Duisburg, Herne and Nörvenich, near Düren, he added.

Police said several apartments were also searched.

Four people have been arrested in total over the alleged plot, which police said was planned by an Islamist terrorist group.

Tip-off led to Christmas Eve arrest

On Christmas Eve, a 30-year-old Tajik national was held in the city of Wesel, some 100 kilometers north of Cologne.

He was suspected of having visited the cathedral possibly to plan an attack.

Hermanns said the first suspect was part of a larger network that extended to other German states and European countries.

North Rhine-Westphalia's Interior Minister Herbert Reul told the media briefing that the police operation had been a "success."

He added that Islamist extremists are currently more active than usual and that "the police always try to be a few steps ahead."

Cologne, other cities step up security

Cologne police said they were deploying around 1,000 officers on New Year's Eve to ensure the festivities pass off peacefully.

Previously, authorities have warned that officers could carry submachine guns around the city on Sunday night.

The underground car park at Cologne Cathedral was searched Sunday and nothing was found, they said.

Despite the tensions, the Cathedral held its New Year's Eve service on Sunday night as normal.

Other German cities also heightened security, including the capital Berlin.

Police there said they were keen to avoid a repeat of last year's violence when some revelers attacked emergency services personnel with fireworks.

mm/dj (AFP, dpa)