The spokesman of the Islamic State group emerged from nearly six months of silence on Monday to mock America's assertion of having defeated the group and to call for retaliation over last week's mosque attacks in New Zealand.

"The scenes of the massacres in the two mosques should wake up those who were fooled, and should incite the supporters of the caliphates to avenge their religion," the spokesman, Abu Hassan al-Muhajir, said in a 44-minute audio recording.

Alleged gunman Brenton Tarrant has been charged over the mass shooting in Christchurch. Credit:AP

Al-Muhajir portrayed the shootings by a white extremist, which killed 50 Muslims as they prayed in the city of Christchurch, as an extension of the campaign against the Islamic State. He likened the mosque attacks to the weeks-long battle raging in the last village under Islamic State control in Syria.

"Here is Baghuz in Syria, where Muslims are burned to death and are bombed by all known and unknown weapons of mass destruction," he said, painting the people in the town as regular Muslims when in fact coalition officials believe that the majority of them are either Islamic State fighters or their wives and children.