ISIS spokesman Abu Hassan al-Muhajir broke a six-month silence to deliver the message in a 44-minute audio recording, according to The New York Times.
Dozens of people were killed when a terrorist went on a rampage through two mosques in the city of Christchurch on March 15.
Australian Brenton Tarrant, 28, is accused of the shooting and has been charged with murder.
Police have said they are certain Tarrant was the only gunman but are still investigating whether he had support from others.
“The scenes of the massacres in the two mosques should wake up those who were fooled, and should incite the supporters of the caliphate to avenge their religion,” al-Muhajir said.
The leader of the terror group compared the Christchurch killings to the battle raging over the village of Baghouz—the extremists’ last stronghold in Syria.
“Here is Baghouz in Syria, where Muslims are burned to death and are bombed by all known and unknown weapons of mass destruction,” he said.
The true identity of the ISIS spokesman is not known—Abu Hassan al-Muhajir is an alias. He is not known to have appeared in any photographs or ISIS propaganda videos. He communicates rarely. His last message—a four-minute long audio recording—was in September 2018.
The attacks in Christchurch left 50 dead and 50 more injured. Nine of the wounded are in critical condition.
After days of intense grieving, preparations were underway to begin burying the victims.
New Zealand Holds First Funerals
After days of preparations, the first funerals took place March 20 for victims of the shootings.
A father and son who fled the civil war in Syria for “the safest country in the world” were buried before hundreds of mourners.
The bodies of Khalid Mustafa, 44, and Hamza Mustafa, 15, were laid to rest five days after the massacre.
Those present included Hamza’s younger brother, 13-year-old Zaed, who was wounded in an arm and a leg during the attack. The boy tried to stand during the ceremony but had to sit back in his wheelchair, one mourner said.
“We tried to not shake his hand, and not touch his hand or his foot, but he refused, he wanted to shake everybody’s hand, he wanted to show everyone that he appreciated them. And that’s amazing,” said Jamil El-Biza, who traveled from Australia to attend the funeral.
The Mustafas had moved to New Zealand last year, after spending six years as refugees in Jordan. Mustafa’s wife, Salwa, told Radio New Zealand that when the family asked about New Zealand they were told, “it’s the safest country in the world, the most wonderful country you can go … you will start a very wonderful life there.”
She added, “But it wasn’t.”
The victims, killed during Friday prayers, were largely Muslim migrants, refugees, and residents from countries including Pakistan, Bangladesh, India, Turkey, Kuwait, Somalia, and others.
A gunman broadcast live footage on Facebook of the attack on one of the mosques, showing him driving to the scene, entering the building and shooting people at random.
Witnesses described the shooter as dressed in a bulletproof vest and a helmet as he opened fire at the Masjid Al Noor mosque at 1:40 p.m. local time, where there could have been as many as 300 people praying.
“He had a big gun … he came and started shooting everyone in the mosque, everywhere,” said witness Ahmad Al-Mahmoud, according to Reuters. He said he and others escaped by breaking through a glass door.
Video footage—not corroborated independently for authenticity—shows worshippers, possibly dead or wounded, laying huddled on the floor. After spending more than two minutes inside the mosque spraying people with bullets, the gunman then walks outside to the street, where he shoots at people on the sidewalk.
Children’s screams can be heard in the distance as he returns to his car to get another rifle.
The gunman then walks back into the mosque, where there are at least two dozen people lying on the ground. After walking back outside and shooting a woman there, he gets back in his car and drives away. The video then cuts out.
Witness Len Peneha told The Associated Press he saw the gunman enter the mosque and then heard dozens of shots, followed by people running from the mosque in terror.
Peneha, who lives next door to the mosque, said after the gunman drove away, he went into the mosque to try and help.
“I saw dead people everywhere. There were three in the hallway, at the door leading into the mosque, and people inside the mosque,” he said. “It’s unbelievably nutty. I don’t understand how anyone could do this to these people, to anyone. It’s ridiculous.”
Hospitals said children were among the victims.
Reuters and The Associated Press contributed to this report.