Austria on Sunday said three people were detained for suspected involvement in an "Islamist network", as Vienna and Cologne step up police controls citing heightened security concerns during Christmas.
German police are carrying out strict controls at the entry of the cathedral in the western city of Cologne following a "danger warning" for New Year's Eve.
Austrian police have also stepped up checks, especially around churches, religious events and Christmas markets in Vienna, citing an "increased risk".
Austrian authorities "intervening in an Islamist network" made four arrests Saturday, the interior ministry said Sunday.
A ministry spokesman said three of the four had been detained pending further investigations.
"There was no immediate threat of an attack in Vienna," he told AFP, declining to comment further.
According to the German daily Bild, one arrest was also made in Germany.
The arrested suspects are said to be Tajiks who allegedly wanted to carry out attacks for Islamic State-Khorasan (IS-K), an IS offshoot in Afghanistan, Bild reported.
"Terrorist actors across Europe are calling for attacks on Christian events -- especially around December 24th," Vienna police said in a statement.
German Interior Minister Nancy Faeser pledged a firm response to any extremist threat.
"We all love our Christmas traditions and won't allow ourselves to be intimidated or have our way of life hemmed in," she told regional media group Funke.
However "we take the danger of Islamist terror very seriously and are extremely vigilant", she said. "Our security authorities have their sights on the Islamist scene and are acting decisively as the current measures show."
Besides Austria and Germany, Spain also received indications that an Islamist group was planning several attacks in Europe, possibly on New Year's Eve and Christmas, Bild said.
According to the newspaper, the targets of these attacks could be Christmas masses in Cologne, Vienna and Madrid.
In July, a cell of suspected Islamists from IS-K was dismantled in Germany and the Netherlands.
Those arrested in Germany were said at the time to have been planning an attack in the country.
