An alleged terror suspect is led by police at the Federal court in Karlsruhe, Germany, Thursday, June 2, 2016. Prosecutors said three Syrian men suspected of planning an attack in Duesseldorf for the Islamic State group have been arrested in Germany. They say a fourth suspect, who informed officials in Paris about the plot, was already in custody in France. (Uli Deck/dpa via AP)

BERLIN - Authorities in Germany are monitoring almost 500 Islamic extremists they believe pose a potential security threat, officials said Friday, a day after the arrest of three men suspected of planning to carry out an attack in the country for the Islamic State group.

The men — identified only as Syrian citizens Hamza C., 27; Mahood B., 25; and Abd Arahman A. K., 31 — were arrested in three separate locations across Germany following a tip from a fourth suspect who had approached French authorities earlier this year.

While Germany hasn't suffered mass-casualty attacks by Islamic extremists of the type seen in France and Belgium over recent months, authorities say the country is a target and the risk of attacks is high.

"It's definitely too soon for a fundamental reassessment in light of what happened yesterday," Interior Ministry spokesman Johannes Dimroth said, adding that authorities will have to await the outcome of prosecutors' investigation.

All three of those arrested in Germany were living in refugee shelters, adding to concerns that IS might be sending fighters to Germany disguised as asylum-seekers. Almost 1.1 million people were registered as asylum-seekers in Germany last year, many of them fleeing war and persecution in Syria, Iraq and Afghanistan.

Dimroth said authorities had received regular tips about possible Islamic extremists coming to Germany as asylum-seekers and were systematically checking those reports. He didn't say how many of the 499 Islamic extremists under observation by state and federal police were asylum-seekers.

Federal prosecutors say the men who were arrested intended to carry out an attack in the western city of Duesseldorf though they had no concrete plans.

German weekly Der Spiegel reported Friday that the plot was to involve a total of 10 attackers, of whom two were to detonate suicide vests. The magazine didn't provide a source for that information, which it said French authorities received from Saleh A., a 25-year-old Syrian in custody in France. None of the suspects' surnames were given due to German privacy rules.

Authorities spent months quietly tracking the three men before swooping Thursday because one of the suspects planned to travel to southern Europe, Der Spiegel reported.

Overall in Germany, Justice Ministry spokesman Philip Scholz said federal prosecutors are currently conducting nearly 120 investigations involving over 180 suspects in connection with terrorism offences.

Wolfgang Bosbach, a senior member of Chancellor Angela Merkel's party, warned Friday against placing all refugees under suspicion.

But he told public broadcaster rbb-Inforadio it was important to know exactly who was coming to Germany.

News from © The Associated Press, 2016