A morning radio show in Sweden was suddenly interrupted by ISIS propaganda after unknown assailants hijacked the ether on Friday morning.

Mix Megapol, which is one of Sweden's most popular commercial radio stations, had finished playing Ed Sheeran and Carly Rae Jepsen when it began broadcasting an ISIS recruitment song.

Thousands of listeners in southern Sweden were subjected to 'For the sake of Allah' - a song ISIS uses to recruit fighters - for up to 30 minutes.

'That was Ed Sheeran, and now.. ISIS!': Swedish radio was interrupted by 30 minutes non-stop repeat of an ISIS recruitment song on Friday morning

Mix Megapol have confirmed that their broadcast was hijacked by a pirate transmitter for around 30 minutes, beginning at 8.30am CET.

The song 'For the sake of Allah' is around three minutes long, and had been played on repeat until Mix Megapol could retake the air.

'This is of course very serious, particularly because they had been playing a song about ISIS recruitment,' a spokesperson told 24Malmo, who broke the news.

'We see this as a very serious incident, and have therefor reported it to the police'.

Recruiting: The song 'For the sake of Allah' has been used by ISIS to recruit foreign nationals to come and fight in Iraq and Syria

As the pirate hijacking only appeared locally in Malmo, Sweden's third largest city, the incident was not discovered until concerned listeners phoned the station.

It is not know if the pirate transmitter was operated by anyone affiliated with ISIS, or if the hijacking was intended as a prank.

Some 300 people have left Sweden to fight for ISIS, according to Swedish Security Service.

This puts Sweden in second place after Belgium for ISIS fighters per capita who have left their home countries for Iraq and Syria.