Denmark's government said Friday it wanted to ban travel to areas where terror groups were operating in conflict zones in a bid to facilitate the prosecution of fighters returning from Syria.

The move came in the same week that five men were detained in Denmark on suspicion of having joined the Islamic State jihadist group while in Syria.

"It will simply be prohibited to set foot in certain areas," Justice Minister Soren Pind said in a statement.

"That way we make it easier to punish people who travel to, for example, Syria to join or support a terror organisation," he added.

Under the proposal, Danish citizens and residents would face up to six years in jail if they traveled to an area where a "terror organisation" is taking part in an armed conflict.

The maximum penalty for "letting oneself be recruited to commit acts of terrorism" would also be raised to 10 years from six, and to 16 years for those who have taken part in combat operations.

Some groups of people, such as journalists and those working for public authorities, could be exempt from the ban.

The opposition Social Democrats and the anti-immigration Danish People's Party said they would back the minority government's proposal, which would give it majority backing in parliament.

The proposal is to be put before parliament in the coming months and, if approved, will go into force on July 1.

But it has been criticised by experts who said it was unlikely to deter radicalised youth from travelling to Syria.

"Those who want to fight for the Islamic State group will be indifferent to any prison sentence," Tore Hamming, an expert on militant Islamism at the European University Institute in Florence, told Danish news agency Ritzau.

"They already lack confidence in the Western state model or our laws," he added.

Since March last year, Danish authorities have been able to seize passports and issue travel bans belonging to those suspected of planning to participate in armed conflict abroad.

Four men in the Copenhagen area were on Thursday detained on suspicion of joining the Islamic State group while in Syria, and were on Friday placed in pre-trial detention by a district court.

A fifth man accused of committing the same crime was jailed by the court on Tuesday, but his detention had been kept secret so as not to interfere with the ongoing investigation, police said on Friday.

Another four men, also accused of joining IS, were jailed in absentia by the Frederiksberg court, but public broadcaster DR said they were believed to still be abroad.