German police on Thursday arrested two Algerians suspected of links to the Islamic State group and hunted two others after raids targeting several sites including refugee shelters where some of the suspects lived.
The four "from the jihadist scene are under investigation over suspicions that they are planning a serious act threatening the security of the state", Berlin police said.
The alleged involvement of Algerian nationals in any IS plot as well as a link to refugee shelters is expected to add fuel to a raging debate over the 1.1mn asylum seekers that Germany took in last year.
North African migrants were already in the spotlight after they were blamed for a rash of sexual assaults during New Year Eve's festivities in the western German city of Cologne.
But the latest arrests risk compounding fears that jihadists are taking advantage of the massive influx of asylum seekers to Europe to slip in undetected.
The operation came as a huge police deployment was underway in Cologne to avoid a repeat of the New Year rampage during the giant week-long carnival festivities which begin on Thursday.
One of the two men captured on Thursday is sought by Algerian authorities for his links to IS, police said, adding that "investigations show that he has been trained militarily in Syria."
The second Algerian was arrested for having falsified identity documents.
A woman was also detained in the course of the raids, although police did not specify the reason.
Some 450 officers took part in the operation sweeping Berlin, and the states of North Rhine-Westphalia and Lower Saxony, and carried away computers and mobile telephones.
Among locations searched were "the refugee shelters where the suspects lived," Berlin police spokesman Stefan Redlich told news channel N24.
In Berlin, police officers were seen taking away a man who had his head covered with a towel, according to an AFP photographer at the scene. It was not immediately clear if this was an additional arrest.
Berlin's home affairs chief Frank Henkel said the "level of threat posed by Islamist militants remains high".
"We have every reason to remain vigilant and careful. Therefore there should be a systematic crackdown on the Islamist scene, especially if there are likely links to IS," he said.
Since the November Paris attacks in which IS gunmen and suicide bombers attacked Paris nightspots, killing 130 people, German authorities have issued terror alerts on several occasions.
In November, a Germany-Netherlands football match was called off and on New Year's Eve, several Munich railway stations were closed over such threats.
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