Berlin (AFP) - German police Thursday arrested three Algerians suspected of links to the Islamic State group after raids targeting several sites, including refugee shelters where some of the suspects lived.

In all, four Algerians "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 group was suspected of planning a possible strike against the German capital, a spokesman for Berlin prosecutors told AFP.

The alleged involvement of Algerians in an IS plot and the link to refugee shelters is expected to add fuel to a raging debate over the 1.1 million asylum seekers that Germany took in last year.

North African migrants have already been 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 slip into Europe undetected.

The operation came as a huge police deployment was underway in Cologne to avoid a repeat of the New Year's Eve attacks during the giant week-long carnival festivities which begin Thursday.

One of the two men captured is wanted by Algerian authorities for his alleged links to IS, police said, adding that "investigations show that he has been trained militarily in Syria."

The suspect and his wife, who was also detained and sought by Algiers over alleged connections with IS, lived in a refugee shelter in Attendorn, about 80 kilometres (50 miles) from Cologne.

The second Algerian man was arrested on suspicion of falsifying identity documents.

Police officers tracked down two other suspects but did not arrest them.

National news agency DPA said one of the Algerians had contact with Belgium's Islamist movement and had travelled at least once in recent weeks to Molenbeek, the troubled Brussels neighbourhood which has emerged as a hotbed of extremism.

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Berlin police said however that they were still investigating the link.

- 'Threat remains high' -

Some 450 officers took part in the operation in Berlin and the states of North Rhine-Westphalia and Lower Saxony, and seized computers and mobile telephones.

Among the locations searched were "the refugee shelters where the suspects lived," Berlin police spokesman Stefan Redlich told news channel N24.

He also told a news conference that two of the four suspects had sought to pass themselves off as Syrians, who benefit from more liberal asylum policies than other nationalities.

The two suspects targeted in Berlin were however not asylum seekers, had jobs and had been living in the capital for a while, police said.

No weapons were found in the operation, Redlich said, but mass circulation daily Bild said that investigators were searching for explosives.

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 attacks in Paris, which killed 130 people and were claimed by IS, 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.