Nine mosques across the Netherlands received hate-filled letters smeared with what appeared to be blood, alarming Muslim communities and prompting outrage, local media reported Friday.

The mosques targeted are located in Rotterdam, Eindhoven, Arnhem, Tilburg, and The Hague, according to Dutch daily AD.

The letters contained defamatory language, insults, and cartoons of the Prophet Muhammad - seeking to insult by flouting Islam’s ban on such representations.

"This is a form of intimidation and threat,” Joram van Klaveren of the national mosque association K9 told public broadcaster NOS.

"The letters are full of hate rhetoric. In Rotterdam, one letter even referred to the ‘last days of Islam in Europe’.”

Van Klaveren said the incident evoked painful memories of past violent attacks on Muslims, including the 2019 terrorist assault on two mosques in Christchurch, New Zealand, in which 51 people were killed, and the murder of a worshipper during prayers in France earlier this year.

"We hope this is not a prelude to something worse, but you never know in these times,” he said.

"Often it starts with intimidation, phone calls, letters. These letters contained deeply disturbing texts.”

The mosque federation said Muslim communities in the Netherlands regularly face harassment, such as verbal abuse and spitting directed at worshippers outside mosques.

Mosque leaders are in contact with local municipalities about possible safety measures. In Rotterdam, immediate discussions were held with Mayor Carola Schouten, Van Klaveren said, adding: "We have asked whether new security measures can be taken, and authorities are now looking into that.”