"We will be here for the next 10 years. Donors need to stay the course. It's a marathon," Jan Egeland told AFP by telephone from Maiduguri in northeast Nigeria.
"(The Boko Haram conflict) is now in its ninth year. That's two years more than the Syrian war, double the First World War. This is a long-term thing."
Egeland was speaking as the NRC unveiled new research about the intentions of the 1.8 million people who are still internally displaced in remote northeast Nigeria.
Of 27 000 people surveyed, 86% said they were not ready to return to their homes while 60% said insecurity was their main reason for staying put for now.
Boko Haram's quest to establish a hardline Islamic state in northeast Nigeria has left at least 20 000 dead and threatened security across the wider Lake Chad region.
The state government in Borno, of which Maiduguri is the capital, has estimated damage to homes, schools, businesses, power, water and other infrastructure to be worth $5.9bn.
Since early 2015, counter-insurgency operations comprising troops from Nigeria and its neighbours Cameroon, Chad, Niger and Benin have pushed Boko Haram out of captured towns and villages.
But attacks - particularly suicide bombings - remain a threat to civilians, including farmers in the mainly rural northeast who are trying to return to their fields.
In early September, jihadist fighters even fired a rocket-propelled grenade into a camp for internally displaced people (IDP) protected by the military at Ngala, near the border with Cameroon.
Seven people were killed and several others injured.
'Here for the long haul'
Egeland said emergency assistance from local and international aid agencies had transformed the northeast over the last year, averting a much-feared famine.
But hundreds of thousands of children are still fighting severe acute malnutrition, even after repeated appeals for international funding to run feeding and other programmes.
Aid agencies scrambled to tackle a major outbreak of cholera in the camps in September.
Egeland said it was "mind-boggling" that people would prefer to stay in IDP camps or with distant family and friends rather than return home because of fears for their own safety.
"There's not a single person (left) in the areas where they have their ancestral lands. If there is anyone there, it's Boko Haram coming and making life impossible," he added.
"The authorities have to make it safe for return and we, as humanitarian organisations, have to make it possible to rebuild houses, lives, schools, clinics."
The NRC secretary-general welcomed the fact that a proposal to close the IDP camps in Maiduguri had been dropped and stressed that people should only return home voluntarily.
Cameroon has been criticised for forcibly returning about 100 000 Nigerian refugees in breach of international agreements, putting them at risk from Boko Haram violence.
Egeland accepted that aid agencies will have to be on the ground longer and adapt their approach from emergency assistance to rebuilding livelihoods and development.
"We are here for the long haul," he added.