Said people would be decapitated if they didn't convert to Islam

Police in Sweden are investigating chilling letters posted through letterboxes across the country threatening to behead people unless they convert to Islam.

The notes, which are signed ISIS and carry the terrorist organisation's flag, threaten to decapitate 'non-believers' unless they become a Muslim or pay a religious tax.

They have been distributed among several cities including Stockholm and warn 'the police will not save you from being murdered'.

A copy of one of the ISIS-branded letters posted through doors in several cities in Sweden

The letters, written in Swedish, threaten to behead anyone who does not convert to Islam or pay a special tax

Translated, the notes said: 'In the name of Allah, the merciful, full of grace. You who are not believers will be decapitated in three days in your own house. We will bomb your rotten corpses afterwards.

'You must choose between these three choices: 1. Convert to Islam. 2. Pay the jizya [religious tax] for protection. 3. Or else, you will be decapitated.

'The police will not prevent or save you from you being murdered. (Death comes to all of you).'

Swedish radio stations reported police were investigating the new threats and comparing them to previous warnings of violence.

The notes promised attacks within three days and were posted last Thursday (December 10), in areas including Ronneby, Sigtuna and Västerås with officers on patrol in the latter over the weekend to ensure no one acted on them.

Emil Andersson, police chief in Sigtuna, around 30 miles north of Stockholm, told Radio Uppland the threat was being taken seriously and officers were increasing security measures.

He added the Swedish intelligence agency was also involved in the investigation and examining the symbols and language used in the letters.

Swedish people have previously held demonstrations, pictured, in solidarity with refugees fleeing Syria

More than 200,000 refugees have entered Sweden this year with many heading to Stockholm, pictured

Tensions have been mounting in Sweden over the influx of asylum seekers from Syria, with more than 200,000 arriving this year.

Last month authorities brought in border checks to check the identities of all entering refugees.

Meanwhile in October, members of the Assyrian community in Gothenburg were targeted with sinister graffiti daubed on restaurants and businesses.

Markus Samuelsson, one of 3,000 Assyrian Christians living in Gothenburg, said he found the walls of his restaurant covered with the messages 'convert or die' and 'the caliphate is here'.