Afghan media reports say a suicide bomber attacked a religious gathering on November 20 in Kabul, killing at least 50 people and wounding 83 others who had gathered to mark the birth anniversary of the prophet Muhammad.

The blast happened at the Uranus Wedding Hall and the victims included religious delegates from various parts of Afghanistan, Interior Ministry spokesman Najib Danesh was quoted as saying by TOLONews.

He said the gathering was convened by the Afghan Ulema Council, the country’s largest religious body, and was attended by hundreds of people.  

In a statement, President Ashraf Ghani described the attack as “unforgivable and a clear act of hostility against Islam’s teaching.”  He declared Wednesday a national day of mourning and ordered that flags be flown at half-staff.

The High Peace Council also condemned the attack and said Afghan people will work for peace and stability despite all the challenges they may face in this way.

The United Nations Assistance Mission in Afghanistan (UNAMA) said it was outraged by the attack.

“UNAMA outraged by #Kabul bombing when communities across #Afghanistan are marking day of special religious significance. Credible reports of heavy civilian casualties.  UN human rights teams establishing facts. UN family extends deepest condolences to the many families affected,” UNAMA tweeted.

According to The Washington Post, a police spokesman said that police had not been asked to provide security for the event and the suicide bomber ha been able to slip into the hall undetected.

No group immediately claimed responsibility for the attack.

But suspicion immediately fell on the Islamic State (IS) terror group, which is considered even more ruthless than the Taliban insurgency that has been battling the Afghan government and its foreign backers for 17 years.

The Taliban “strongly condemns attacks on civilians and sessions of ulema,” or Muslim clerics, Taliban spokesman Zabiullah Mujahid said in a statement sent to reporters, according to The Washington Post.

Without naming any group, but alluding to the Islamic State, he blamed the attack on “seditious circles” that he said have brutally killed Muslims over minor differences. He said this must be prevented.