German police are investigating 40 cases in which Islamic militants are suspectedof having entered the country disguised as refugees.

The number of cases being investigated has more than doubled since January, federal police said on Wednesday.

Authorities are also concerned about the large number of refugees and migrants who have been able to seek asylum in Germany without proof of ID.

The number of cases involving suspected ISIS militants having entered Germany disguised as refugees has more than doubled since January, federal police said Wednesday

In the past, the German government has played down the risksof ISIS fighters entering Europe with the tide ofmigrants, in part to avoid exacerbating public concerns aboutthe influx, which hit a record 1.1 million last year.

But Germany's domestic intelligence agency has warned that ISIS appears to have sent fighters 'the migrant route' via Greece, in order to fan fears about refugees and 'send a political signal' - despite there being more efficient ways to smuggle in militants.

'I am not telling you a secret when I say that I amconcerned about the high number of migrants whose identities wedon't know because they had no papers when they entered thecountry,' the head of Germany's domestic intelligence agency Hans-Georg Maassen told a conference last week.

The number of migrants entering Germany reached peaks ofmore than 10,000 a day last autumn, but has fallen dramaticallyin recent months due to the closing of the Greek border withMacedonia and a deal between the European Union and Turkey thathas discouraged refugees from crossing the Aegean Sea.

The reduction in the numbers has eased pressure on GermanChancellor Angela Merkel, who came under fierce criticism lastyear for welcoming hundreds of thousands of migrants fleeing warin the Middle East with the optimistic slogan 'We can do this'.

The head of Germany's domestic intelligence agency Hans-Georg Maassen told a conference last week that although there are more efficient ways to smuggle in fighters, ISIS has sent some via Greece and the Balkans

A spokeswoman for the Bundeskriminalamt (BKA), or federalpolice, said there had been 369 warnings about possibleextremists entering the country since the influx of refugeesaccelerated last year, of which 40 merited further investigationby federal and state authorities.

That represents a sharp increase from the 213 warnings and18 investigations that the police had recorded in early January.

'German security officials have indications that members andsupporters of terrorist organisations are being smuggled in withrefugees in a targeted, organised way in order to launch attacksin Germany,' the BKA spokeswoman said.

Two of the suicide bombers from the November 13 attacks in Paristhat killed 130 people came into Europe through the Balkan routeand so did two men who authorities believe were meant toparticipate in those attacks but were delayed and arrested in arefugee centre in Salzburg in December.

There is also evidence that Saleh Abdeslam, believed to bethe lone surviving suspect from the attacks, picked up threeunidentified militants who entered Europe with the refugees inthe southern German city of Ulm in October of last year.

In early February, German authorities arrested a 35-year-oldAlgerian man and his wife at a refugee centre in the town ofAttendorn.

The man, a suspected ISIS member, reportedlyposed as a Syrian when he entered Germany in the autumn of 2015.