OSLO (Reuters) - Three men were accused in a Norwegian court on Tuesday of joining or aiding the Islamic State in Iraq and Syria (ISIS), in the first case of its kind in the Nordics.

Djibril Bashir and Valon Avdyli pleaded not guilty to participating in a terrorist organisation, and Visar Avdyli denied providing aid to such a group - both offences under a new Norwegian law designed to crack down on returning militants.

Authorities in Norway, Sweden, Denmark and Finland estimate that 300 to 400 people may have left for places like Iraq and Syria to receive militant training. Western powers fear radicalised fighters may come back to launch attacks at home.

All three Norwegian nationals acknowledged travelling to join ISIS, which has grabbed large parts of Syria and Iraq and, analysts fear, opened a new front in Libya.

If found guilty, they face up to six years in prison.

Neighbouring Denmark, where a man with Palestinian roots killed two people in separate attacks 10 days ago, has tried what it calls a soft approach, reintegrating foreign fighters through counselling and assistance in finding housing and work.

Although fighting for ISIS is illegal in Denmark, current laws make it very difficult to prove crimes abroad.

Sweden has also not charged anyone for fighting abroad but is now considering new legislation to make it easier for authorities to prosecute.