A mysterious group that calls itself "Christian State'' has threatened to kill Muslims and destroy their establishments in Belgium, apparently in retaliation to the Islamic State (ISIS) attacks in Paris that killed 130 people on Nov. 13.

The warning was written in a letter delivered last week to the Attadamoune mosque in Molenbeek, Belgium, according to French and Belgian media reports.

"No mosque and none of your businesses will be safe,'' and "brothers [Muslims] will be slaughtered like pigs and crucified as our Lord converts their souls,'' the group said in the letter that was found in the mosque's mailbox by Jamal Habbachich, one of the local Muslim leaders and the president of Molenbeek's mosque association, Rawstory reported.

It also warned that the group " will avenge our brothers who fell in the various [Paris] attacks."

"In the current climate, with fear in everyone's minds, it is disturbing," Habbachich told Le Parisien in an interview. A video posted by Le Parisien shows Habbachich printing a copy of the letter, marked by the initials E.C., or "Etat Chrétien" (Christian State).

"There are two situations when you receive this kind of letter. It is a document written by someone unbalanced, or it is a very serious threat. What also concerns me is the name of the author of the letter, which uses similar terminology to that of the Islamic State,'' he told Belgium's RTBF broadcaster.

The Attadamoun Mosque in Molenbeek is reportedly in the hometown of the three Abdeslam brothers linked to the attacks in Paris. Salah Abdeslam, 26, remains at large; Brahim, 31, killed himself when he detonated a suicide vest at a Paris restaurant. The third brother, Mohamed, was detained by Belgian police but was later released.

Habbachich said he reported the death threat to the police and subsequently filed a complaint to Belgian authorities.

To secure Muslims especially those attending Friday prayers, he also requested police patrols at all of the city's mosques.

Two other mosques in the region also received similar threats and similar letters, local media reported.

The Attadamoune mosque is considered a key Muslim institution in the Molenbeek district. The Molenbeek's mosque association includes 16 to 22 mosques in Brussels.

Last week, Belgian Prime Minister Charles Michel said the government may close "certain radical mosques" in the Molenbeek district over fears that it was the location where the Paris attacks were launched.

On Thursday, Brussels' Grand Mosque, which Saudi Arabia gifted to Belgium, was evacuated after packets of white powder caused a security alert. Authorities later revealed that the powder was flour.

On Wednesday, the four-day lockdown of the European capital came to an end as Belgian authorities lowered the maximum security threat level to "serious" due to fears of a Paris-like attack.