By Garba Mohammed and Mike Oboh
KADUNA/ABUJA, June 20 (Reuters) - At least 80 people have been killed since Monday in clashes in northern Nigeria triggered by Islamists waging an insurgency against the government, figures from police and the Red Cross showed on Wednesday.
The violence - some of which was sparked by church bombings over the last three Sundays - has heightened sectarian tensions in Africa's most populous country, which is evenly split between Christians and Muslims.
Boko Haram insurgents waged gun battles with security forces in the remote northeastern city of Damaturu, near the radical sect's heartland, throughout Tuesday, police chief for the surrounding Yobe state Patrick Egbuniwe told Reuters.
He said 40 people were killed, 34 insurgents and six security personnel.
In separate clashes between Muslim and Christian residents of the northern city of Kaduna on Tuesday, at least 40 people were killed and 62 wounded, according to local Red Cross official Awwal Sani.
His organisation was helping collect bodies and treat the wounded, following riots in which Muslim youths fired AK-47 rifles, burned tyres and destroyed a church in Kaduna.
The riots came two days after after Christian youths went on the rampage, killing 52 people in the city, itself retaliation for the bombing of three churches by suspected Islamists on Sundat that killed 19 people.
Residents said the violence in both cities, hundreds of kilometres (miles) apart, had died down on Wednesday.
"Damaturu is calm this morning. Four policemen are receiving treatment for gun shot (wounds)," police chief Egbuniwe said. "We made seven arrests and they are with the criminal investigation department."
Pope Benedict repeated his concerns about the sectarian killings, using his weekly general audience on Wednesday to appeal for an immediate end to "terrorist attacks" against Christians and urging all sides to avoid reprisals.
Boko Haram says it is fighting to reinstate an ancient Islamic caliphate in the north of Africa's top oil producer that would impose strict sharia or Islamic law. The insurgents have killed hundreds since they launched an uprising in 2009.
They mostly target security forces or authority figures but in the past year have turned their sites on Christian worshippers, attacking churches in an apparent attempt to stoke a wider sectarian conflict.
In November, 65 people were killed in attacks claimed by Boko Haram on churches, mosques and police stations in Damaturu, where security forces often clash with Islamists in gun battles.
Boko Haram claimed responsibility for church attacks on the first two Sundays of this month but has yet to do so for the most recent.
Muslims and Christians that make up most of Nigeria's 160-million population mostly live side by side in peace, but there have been occasional bouts of sectarian violence since independence from Britain in 1960. (Writing by Tim Cocks; Editing by Robin Pomeroy)