German police arrested three people on Sunday over an alleged attack plot targeting the cathedral in Cologne on New Year's Eve.
The "alleged means of attack" is a car, said police in the western city, adding that security measures have been stepped up around the site.
The three suspects are believed to be linked to a Tajik who was arrested on Christmas Eve, said Cologne police chief Johannes Hermann.
The Tajik was detained by German police on the same day as Austria announced the arrests of another three suspects in Vienna.
Bild daily had reported then that the four are all Tajiks who allegedly wanted to carry out attacks for Islamic State-Khorasan (IS-K), an IS offshoot in Afghanistan.
"Islamist people and groups" are "more active than ever at the moment," warned Herbert Reul, interior minister of North Rhine-Westphalia state, where Cologne is located.
Investigations following the Tajik's arrest a week back had found that there is a plot to deploy a car as a means of attack, but "in which way is not known to us," said Frank Wissbaum of Cologne police.
Officers had deployed sniffer dogs to search the underground parking of the cathedral for explosives but they have not yet turned up anything suspicious.
Nevertheless, protective measures have been significantly stepped up, with around 1,000 police officers deployed since this afternoon to "protect the cathedral and the population in Cologne city centre".
Reul voiced confidence that New Year's festivities can go ahead.
"I think that people can celebrate calmly in Cologne today," he said.
- Germany on high alert -
Germany has been on high alert in recent weeks over possible Islamist attacks, with the country's domestic intelligence chief warning in late November that the risk of such assaults is "real and higher than it has been for a long time" because of the Israel-Hamas war.
The deadliest attack by Islamist extremists in Germany was carried out by an IS supporter who rammed a truck into a Berlin Christmas market in December 2016, killing 12 people.
Germany issued a ban on Hamas activities and organisations linked to the group in the wake of the militants' attack on Israel that killed around 1,140 people, most of them civilians, according to an AFP tally based on Israeli figures.
Hamas militants also took some 250 hostages, of whom at least 129 are believed to still be held in Gaza.
In retaliation for the deadliest attack in its history, Israel announced it would destroy Hamas and began a relentless bombardment of the Gaza Strip before a ground offensive there.
Gaza's Hamas government says the death toll in the Palestinian territory had reached 21,822, mostly women and children.