COPENHAGEN (AFP) - Danish police Thursday issued an arrest warrant in absentia for a Danish fighter in Syria who posed with severed heads and said another Dane who travelled to Syria had been held.

Copenhagen police said the warrant for the 26-year-old man was issued for condoning acts of terrorism after he posted some photographs on Facebook.

The pictures showed the man "standing in front of some severed heads, and... wearing a shoulder strap with the Arabic text for the Islamic State (ISIS)," a police statement said.

"The pictures appear to be taken in the city of Raqqa in Syria, which is an area controlled by ISIS, where the accused is presumed to have stayed in July 2014 when the pictures were published on his Facebook profile," it said.

The man also uploaded two pictures of ISIS leader Abu Bakr al-Baghdadi, one of which carried the caption: "I sacrifice myself for you."

"It's not a question of mere expressions of sympathy for terrorist organisations but of approval of actual terrorist acts," chief prosecutor Dorit Borgaard said.

An international arrest warrant would also be issued for the man, who according to tabloid Ekstra-Bladet went missing on September 2 last year.

In the pictures he posed in front of a fence "pointing at severed heads attached to spears, as headless bodies hang from the fence. Several pictures give the impression of fresh blood," the newspaper wrote.

Danish police also said a 21-year-old man from Alleroed, around 30 kilometres northwest of Copenhagen, had been arrested on Wednesday on suspicion of spreading "extremist video material" on the internet.

The man "departed in 2013 for Syria, and has since his return to Denmark been in the police spotlight," the regional police said in a separate statement.

Denmark has drawn international attention for offering counselling and mentorship programmes to radicalised youth who travel to Syria.

But it has not until now charged anyone for crimes committed in Syria, and the softly-softly approach has come under fire from the right wing opposition and the anti-immigrant Danish People's Party, which want to see more Syria fighters prosecuted.