Nearly 8,000 Islamic State (ISIS) militants are now inside Germany blending in with the migrants and planning "combat missions" in Europe, the Daily Mail reported, quoting a German federal police chief as its source.

Hans-Georg Maasen, the federal police chief, told the Austrian Press Agency, that the militants, numbering 7,900, have come from the battlefields of Syria and Iraq.

Maasen said some of these militants are recruiting more militants among the migrants. He described the militants as "combat-hardened professionals" more dangerous than those from al-Qaeda.

He pointed out that his office receives at least one credible threat of a planned terrorist activity every week.

Maasen said they have arrested nearly a dozen terrorists with fake Syrian passports that are nearly identical, including the name. Only the photograph is different.

It was earlier reported that eight migrants have reached Europe using documents almost identical to those carried by one of the eight Paris terrorists.

It may be recalled that at least two of the terrorists—who used assault rifles, grenades and bombs in their suicide vests to slaughter 130 people—had reached the French capital via Greece posing as refugees

The passport, found near the body of one of those who participated in the massacre, identified him as Syrian.

It showed that he claimed asylum on the Greek island of Leros last month with the fake Syrian passport with the name of 25-year-old Ahmad Almohammad, according to the Daily Mail.

Serbian police earlier revealed that they had arrested a man carrying a Syrian passport that was almost a carbon copy of the one found on the ISIS bomber's corpse.

Even before the Paris attacks, U.S. Senator Jeff Sessions (R-Alabama) warned about a thriving black market in Syrian passports during a U.S. House hearing. He said the passports can be purchased for as little as $200, according to WND.

A group of U.S. national security experts also believed that "thousands" of ISIS militants are also already in the U.S. spread out in various cities and just waiting for the signal to launch possibly coordinated attacks just like what a few of them did in Paris, WND said.

The Threat Knowledge Group, a training and strategic threat analysis group with links to the Pentagon, issued the report, saying that ISIS has already recruited thousands of supporters in the U.S.—far more than previously thought—through the Internet and social media.