Macedonian police say three Serbian fans have been injured by a group of unidentified assailants ahead of a World Cup qualifier between Macedonia and Serbia.

The men, one of whom was stabbed, were taken to a hospital in Skopje on Tuesday but their injuries were not considered life-threatening.

There have been no arrests and the cause of the attack is unclear.

In a separate incident in Skopje on Tuesday, two Montenegrin men were assaulted in a shopping mall.

Macedonian police have stepped up security in Skopje ahead of the match.