The UN official expressed "grave concern" after the violence "that has caused tens of thousands of innocent civilians to flee their homes."
The clashes in Nigeria's north-east have triggered massive displacement, the majority "converging on already congested camps or sites for internally displaced people in Maiduguri or Monguno town," according to the UN.
“The impact of the recent fighting on innocent civilians is devastating and has created a humanitarian tragedy,” Kallon said after visiting Monguno and Teachers Village camp in Maiduguri.
New and bloody clashes started in December 2018. More than 30,000 internally displaced people have already arrived in Maiduguri, mainly from Baga, in recent weeks, according to the UN:"Tens of thousands of people are in need of humanitarian assistance, notably shelter, food, water and sanitation."
Aggravating the crisis, the largest withdrawal of aid workers since 2016 happened in November. Some 260 aid workers have been withdrawn from three local government areas (Monguno, Kala/Balge and Kukawa) affected by the conflict since November, which has hampered delivery of humanitarian aid to hundreds of thousands of people.
“It is heart-wrenching to see so many of these people living in congested camps, or sleeping outside with no shelter," said Kallon.
"Civilians continue to bear the brunt of the conflict and the United Nations is extremely concerned about the impact that violence in north-east Nigeria, especially in Borno State, is having on civilians."