Norwegian intelligence services announced Tuesday the arrest of three men suspected of wanting to support or join a radical group in Syria.

"There are strong reasons to believe that two of the arrested people have acted as foreign fighters for the Islamic State of Iraq and the Levant (ISIL)," the Norwegian Police Security Service (PST) said in a statement.

The three men were Norwegian citizens residing in the Oslo region and have denied all accusations, PST spokeswoman Siv Alsen told AFP after the questioning.

The identity of the suspects was not revealed, but one of them is of Somali origin and 29 years old, and the others are two brothers originally from the former Yugoslavia, aged 27 and 24.

The siblings had a third brother who recently was killed in Syria, according to PST.

All three suspects were arrested in the morning in their homes "in order to prevent any further support or involvement with ISIL," the intelligence services said.

"We consider them a threat for Norway," PST senior officer Jan Glent told public broadcaster NRK.

According to Norwegian intelligence, at least 40 to 50 people with links to Norway have taken part or still participate in combats against Syrian president Bashar al-Assad.

Norway is concerned that these jihadists will return to their country, toughened and radicalised.

In Syria, ISIL is fought by other rebel groups -- moderate and islamist -- which accuse it of seeking hegemonic power and of committing atrocities against civilians, pacifist activists and rebels.