By Stephanie Nebehay

GENEVA (Reuters) - Nigeria's economic slowdown, compounded by Boko Haram attacks, could mean 5.5 million people needing food aid in the volatile northeast by next month, double the current number, the United Nations warned on Friday.

As government troops advance against the militants, the somewhat better access for aid workers under military escort to Borno and Yobe states has exposed "catastrophic levels" of suffering and a "vast regional crisis", according to the U.N. refugee agency UNHCR.

Inflation and soaring food prices come at a time when people have little left from the last harvest, the U.N.'s World Food Programme (WFP) said.

"Because of Nigeria's economic downturn, the number of hungry people could double in the northeastern states that are already so heavily afflicted by the conflict," WFP spokeswoman Bettina Luescher told a news briefing.

"Our experts are warning it could go as high as 5.5 million people by next month," she said. "The drop in oil prices and sharp rise in the cost of imported staples has compounded the years of violence that these poor people had to suffer."

WFP has delivered food to 170,000 people in northeastern Nigeria, but hopes to reach 700,000 by year-end, Luescher said. It is also providing aid to 400,000 people in the three other Lake Chad Basin countries - Cameroon, Chad and Niger.

Nigerian Oil Minister Emmanuel Ibe Kachikwu said on Thursday that the OPEC country's crude output had fallen to 1.56 million barrels per day (bpd) as persistent militant attacks have taken out around 700,000 bpd.

Medecins Sans Frontieres (MSF) said in late July that severely malnourished children are dying in large numbers in northeast Nigeria, the former stronghold of Boko Haram militants where food supplies are close to running out. The aid agency warned of "pockets of what is close to a famine".

UNHCR spokesman Adrian Edwards said on Friday the situation remains dangerous and volatile, following an attack on an aid convoy last month. "There have been frequent 'hit and run' incidents by militants, including suicide bombings, attacks on civilians, torching of homes, and thefts of livestock."

Armored vehicles and military escorts are urgently needed to provide protection for aid workers, he said.

"We have seen adults so exhausted they are unable to move, and children with swollen faces and hollow eyes and other clear indications of acute malnutrition," Edwards said.

(Editing by Mark Heinrich)