Mohammed al-Maghayer went missing Tuesday morning and was released several hours later after having been interrogated about his links to the Dutch, the relative said.

The Dutch delegation to the Palestinian Authority in the West Bank had no immediate comment.

A medic at Gaza City's Shifa Hospital treated Maghayer for "wounds to his legs" and he was discharged early Wednesday.

"Mohammed was kidnapped around 8:00 am near his home in Gaza City," the relative, who spoke on condition of anonymity, told AFP.

"His abductors told him they belonged to the “Islamic State” group.

"They put metal handcuffs on him and drove him to an undisclosed location where they held him for about eight hours, asking information about himself and his work with the Dutch."

Maghayer, who said he works for the Dutch as a local advisor on Gaza, told AFP he was "kidnapped and beaten," but made no further comment.

Hamas, which controls the Gaza Strip, said its "security forces have launched an investigation to find who is behind this attack."

In the past few months, statements purportedly from "Gaza Islamists" have been published online pledging allegiance to the ISIS, but Muslim extremists on social media dismissed them as forged.

One such statement threatened "journalists who publish false information about the ISIS."

No known group in Gaza has made such overtures, but experts and even Hamas, which controls the territory, has warned that instability could make it a ripe environment for extremists who identify with the ISIS.