Six men were arrested in Denmark on suspicion of being ISIS members or financiers, police said on Tuesday.
Two were arrested in Copenhagen, the Danish capital, and four in Aarhus, Denmark's second-biggest city, in the west of the country.
"Two of the people arrested, a man aged 29 from the region of Aarhus and a 30-year-old man living in Copenhagen, are suspected of penal code violations ... for having travelled to Syria in 2014, where they were recruited by the terrorist organisation Islamic State," police said.
The 29-year-old suspect was also accused of attempting to return to Syria in early 2015 to rejoin ISIS.
Four of the suspects were accused of acting as "intermediaries" under his instruction and sending money to the extremists.
At least 160 people travelled from Denmark to fight in Syria or Iraq, the Danish intelligence service said.
About a third of them were killed in action, 32 remain in the region and around half of them have returned either to Denmark or gone another country, said the intelligence service.
As the arrests were made public, Denmark announced it would push ahead with plans to return refugees to Syria.
"Denmark has been open and honest from day one,” Immigration Minister Mattias Tesfaye said.
“We have made it clear to the Syrian refugees that their residence permit is temporary and that the permit can be revoked if the need for protection ceases to exist."
On Tuesday, the EU's top migration official expressed concern about Denmark’s decision to review hundreds of residency permits for Syrian asylum seekers.
Sweden's Ylva Johansson, the bloc's migration commissioner, said she raised the matter with Copenhagen, where the government assured her it would not force any deportation.
