The suspects, whose identities were not disclosed, are aged between 27 and 35, police in East Jutland said.
Two of the suspects were arrested in the Danish capital Copenhagen and the four others in Aarhus, Denmark’s second-biggest city.
“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 in a statement.
The 29-year-old is also suspected of having tried to return to Syria in early 2015 to rejoin Isis.
Under his instruction, the four other suspects are accused of having acted as “intermediaries” and having sent money to the organisation.
According to Danish intelligence service PET, at least 160 people have travelled from Denmark to fight in Syria or Iraq. About a third of them have been killed in action, 32 are still there and around half of them have either returned to Denmark or another country.
Jihadism is considered the biggest threat to Denmark’s national security, according to PET.