The UN has been accused of failing to act quickly enough to save hundreds of thousands of lives in northern Nigeria where a food crisis already killing hundreds of people a day is poised to become the most devastating in decades.

Nigerian authorities, who maintain tight control over humanitarian and media access to the region, have also been accused of deliberate negligence and attempting to conceal the scale of the crisis.

The UN Office for the Coordination of Humanitarian Affairs (OCHA) has categorised 4.4 million people in the Lake Chad region as “severely food insecure” – meaning they are in need of urgent food aid.

Toby Lanzer, UN assistant secretary general and OCHA’s regional humanitarian coordinator for the Sahel, said: “This is about as bad as it gets. There’s only one step worse and I’ve not come across that situation in 20 years of doing this work and that’s a famine.”

“We have to step in and quickly or we are going to have hundreds of thousands at risk of dying in the north-east of Nigeria.”

Boko Haram’s seven-year insurgency has left Borno’s farmland – which previously fed Nigeria – devastated and abandoned. This will be the region’s third year without a harvest.

The hunger crisis is claiming lives even in Maiduguri, the capital of Borno state and the hub of humanitarian and security forces in the region. The city has doubled in size in two years and now hosts 2.4 million displaced people. Food prices are soaring in the markets, where it now costs $100 (£75) to buy a large bag of rice.

Lanzer said UN agencies have not had the resources necessary to tackle the crisis and has called on international donors to prevent a greater catastrophe. Of the $279m (£210m) required, only $75m has so far been secured.

Isabelle Mouniaman, head of Médecins Sans Frontières operations in Nigeria, said MSF has been raising the alarm in northern Nigeria for two years and UN organisations have failed to respond.

“We’ve been calling to the UN, to the headquarters of Unicef, WFP [World Food Programme], OCHA and their response has been ‘Yes, we’re doing this and that’… But you cannot just be satisfied to say you built X number of latrines, delivered X bags of food when people are dying. It’s not enough,” Mouniaman said.

“The Red Cross is doing their job, MSF is doing their job, but the vast majority of humanitarian organisations are failing in their responsibility towards the crisis in Borno.”

International aid agencies have focused on Maiduguri’s overstretched camps, but more than 80% of displaced people in the city, around 1.9 million people, are living among the community, the vast majority without access to food aid or medical support.

The most desperate crisis is unfolding outside Maiduguri, where aid agencies fear hundreds of thousands of people are trapped, cut off by Boko Haram and the military operation against them. As the Nigerian army clears more of these areas, the true scale of the crisis is only just becoming clear; those who have escaped tell of watching children die from hunger and being prevented from calling for help.

Mouniaman said: “We’re talking about areas in which 39% of children have severe acute malnutrition. This is a really, really dramatic situation. In my whole MSF career – since 1999 – I’ve never seen anything like it.”

In June, a humanitarian convoy reached Bama, Borno state’s second largest city. It was recaptured by the Nigerian army in March 2015, but the 37-mile journey (60km) from Maiduguri is still considered too dangerous to make without military escort because of Boko Haram attacks and landmines.

They found Bama destroyed and a camp of about 30,000 people, mostly women and children. Many were starving. MSF found the graves of 1,233 who had died in the camp, 480 of whom were children. More than 3,000 severely malnourished people were evacuated by the state governor to Maiduguri for emergency treatment. Several died en route.

The Guardian was refused entry to Bama by the Nigerian military on security grounds. But Maj Gen Leo Irabor, who leads the military operation against Boko Haram in the region, said hunger in the Bama camp was “relative”.

“Very largely I think their needs are being met,” Irabor said.

Several people evacuated to Maiduguri agreed to speak to the Guardian on condition of anonymity. One man, a civil servant, said he had seen people die every day in the camp as a result of hunger and poor sanitation.

Food rations were delivered once a day by civilian militia and distributed by local community heads. This was often raw rice, which there was no means to cook. Complaints about hunger and deaths were ignored.

“How many times we cried out or we complained … But when we were in Banki, the army confiscated all our mobile phones. If the army saw you making a telephone call, wow would they give you a beating,” he said.

Humanitarian agencies are still struggling to get an idea of the scale of need in tens of towns they have not been able to reach. In Mondugo last week, MSF estimated 100,000 displaced people were in need of assistance; this week, their revised estimate was 200,000. There is even less information about large communities in Dikwa, Konduga, Gwoza and Kale/Balge, where the situation is thought to be even worse than in Bama.

Grema Terab, chairman of the State Emergency Management Agency (Sema) in Borno – the body leading the state’s humanitarian response – until early March 2015, believes the crisis is the result of “total neglect and carelessness on the part of the government”. He said the government was aware of the extent of the hunger, but failed to deliver a plan to tackle it and attempted to prevent media coverage of the issue for fear of embarrassment.

“The government chose to conceal the issue of IDPs [internally displaced people] because they were afraid of indictment. There has been a lot of long-term neglect and a refusal to act upon the plight of the IDPs and this is why starvation is occurring in most of the camps,” he said.

“The IDPs are kept under lock and key because they don’t want them to communicate with the outside world.”

The current Sema chairman, Satomi Saleh, told the Guardian these allegations were “blackened lies and political connivances”. He said Sema, alongside the National Emergency Management Agency, has reached 150,000 people in the camps in Maiduguri with food assistance, but admitted the crisis has now exceeded Nigeria’s ability to respond alone.

A nutritional emergency has been declared in Borno state, where the governor, Kashim Shettima, is now working closely with UN agencies. The WFP was invited into Nigeria by the government in March to assist the relief effort. They are rapidly scaling up their operation and now hope to reach more than 700,000 with food aid by December.

“I don’t think anyone was quick enough to understand how serious the situation was. We can criticise each other, but the main point is … what are we going to do to make sure this situation doesn’t deteriorate,” Lanzer told the Guardian.

“We can make every plan on earth … [but] if we do not get resources from the donor community very little of that will actually happen.”