A weekend attack on farm workers in northeast Nigeria blamed on jihadists left at least 110 dead, the UN humanitarian coordinator in the country said on Sunday, the deadliest attack on civilians this year.
The attack, in a state gripped by a jihadist insurgency for more than 10 years, took place the same day as long-delayed local elections in the state.
"I am outraged and horrified by the gruesome attack against civilians carried out by non-state armed groups in villages near Borno State capital Maiduguri," Edward Kallon said in a statement.
"At least 110 civilians were ruthlessly killed and many others were wounded in this attack," he added. Initial tolls indicated 43 and then at least 70 dead from Saturday's massacre.
Some locals blamed the attack on Boko Haram fighters, but Bulama Bukarti, an analyst with the Tony Blair Institute, said rival group the IS-affiliated Islamic State West Africa Province (ISWAP) were more active in the area.
"ISWAP is the likely culprit," he tweeted.
Kallon, in his statement, said: "The incident is the most violent direct attack against innocent civilians this year.
"I call for the perpetrators of this heinous and senseless act to be brought to justice," he added.
The violence centred on the village of Koshobe near the Borno state capital Maiduguri, with assailants targeting farm workers harvesting rice fields. One pro-government anti-jihadist militia said the assailants tied up the labourers and slit their throats.
Kallon said the assailants -- "armed men on motorcycles" -- also targeted other communities in the area.
"Rural communities in Borno State are facing untold hardships," he added, calling for more to be done to protect them and to head off what he said was a looming food crisis there.
Borno Governor Babaganan Umara Zulum attended the burial Sunday in the nearby village of Zabarmari of 43 bodies recovered on Saturday, saying the toll could rise after search operations resumed.
The victims included dozens of labourers from Sokoto state in northwestern Nigeria, roughly 1,000 kilometres (600 miles) away, who had travelled to the northeast to find work, it said.
Six were wounded in the attack and eight remained missing as of Saturday.
Kallon, citing "reports that several women may have been kidnapped", called for their immediate release.
Nigerian President Muhammadu Buhari condemned the attack on Saturday, saying: "The entire country has been wounded by these senseless killings."
Neither the president's statement nor Sunday's from the UN mentioned either Boko Haram or rival group ISWAP by name.
But both groups have been active in Borno State, their attacks having forced the postponement of locations in Borno State, which finally took place Saturday.
The two groups have been blamed for increasing attacks on loggers, farmers and fishermen whom they accuse of spying for the army and pro-government militia.
They have also targetted them for failing to pay a tax they impose on anyone economically active in some parts of the state.
Last month Boko Haram militants slaughtered 22 farmers working on their irrigation fields near Maiduguri, in two separate attacks.
At least 36,000 people have been killed in the jihadist conflict, which has forced around two million people to flee their homes since 2009.
The violence has also spread into neighbouring Niger, Chad and Cameroon, prompting a regional military coalition to fight the militants.