A 21-year-old man arrested in Germany on suspicion of planning an Islamist attack has admitted links to a man being held by the Austrian police for the same reason, German investigators said on Monday.

Police in Vienna said they would remain on high alert until they could rule out that attacks were being planned. It was not clear whether the suspects had been planning separate attacks or a joint one, or what kind of attack that might be.

The German admitted during questioning that the Austrian, a 17-year-old with Albanian roots, had visited him for two weeks at the end of last year, a spokesman for the Duesseldorf prosecutor said.

"We are particularly examining whether there is still a potential danger," Austria's director of public security, Konrad Kogler, told a news conference, adding that the Austrian suspect may have been planning an attack on the underground transport system or other public places in Vienna.

German security police arrested the man and a woman during a raid on a flat in the western city of Neuss on Saturday, the regional Criminal Investigation Office said. The woman, identified by local media as the man's wife, was later released.

German authorities have been on high alert since a Tunisian whose bid for asylum had been rejected rammed a truck into a Christmas market in Berlin on Dec. 19, killing 12 people.

A search of the apartment in Neuss did not turn up any evidence of an imminent attack and no weapons or explosives were found, a prosecutor's spokesman said.

Germany's Focus magazine said the man was planning a bomb attack on police and soldiers. Both he and the Austrian had experimented with materials to create explosives in the Neuss apartment, it said.

No evidence of an attack was found at the Austrian suspect's apartment either but he had expressed support for Islamic State, Austrian Interior Minister Wolfgang Sobotka told ORF radio.

Kogler, the Austrian security chief, said a boy thought to be 12 years old was "under control". Children under 14 cannot be arrested in Austria.

The Neuss arrest followed a tip-off from the Austrian authorities, who had themselves acted on intelligence from other countries.

Vienna police asked people to remain vigilant for suspicious items like abandoned bags.

"I cannot say whether it will be days or weeks", police Vice President Karl Mahrer said when asked how long the heightened alert level would last.