More than 80 percent of people living in a conflict zone in Sudan's southern region are eating only one meal a day, compared with 10 percent one year ago, a U.S. advocacy group said Thursday, citing research collected from a region where aid groups aren't allowed to operate.

The Enough Project warned that hunger is increasing to dangerous levels in the Sudanese state of South Kordofan. A study carried out by an aid group showed that girls are suffering the worst. Girls in the region have reached a "critical" level of malnutrition, the most serious classification on the World Health Organization's scale. Boys are just below that level, at "serious."

The research was conducted by an aid group the Enough Project said didn't want to be identified. Sudan forbids aid groups from operating in South Kordofan, where anti-government rebels are fighting government troops. Aid workers must sneak into the region by crossing over the border with South Sudan.

Thousands of families fleeing hunger and violence in the Nuba Mountains have streamed into South Sudan over the last year.

Health experts at The John Hopkins Bloomberg School of Public Health found the aid group's research and methodology to be credible, Enough said. It called the research significant because it is the first international, on-ground assessment of the food situation in South Kordofan since June 2011, when Sudan banned aid groups from operating in the state.

John Prendergast, co-founder of the Enough Project, said the situation in South Kordofan is similar to the conditions seen leading up to the famine in Somalia last year.

"If the international community does not respond to these early warning indicators in South Kordofan, the situation could have devastating consequences for hundreds of thousands of people. Pre-positioning of relief supplies must begin now, and all possible means of delivery need to be explored and, if negotiations over access fail, utilized," Prendergast said.

Sudan's interior minister on Tuesday said that fighting in South Kordofan and the neighboring state of Blue Nile has killed more than 600 people in the last 16 months. The minister said that the military and police sustained heavy losses in battling the insurgency, but he did not break down the figures between civilian, government, and rebel casualties. A spokesman for the rebels — Sudan People's Liberation Movement-North — told The Associated Press by phone that his group rejected the figures.

The fighting pits the Khartoum government against rebel groups allied with the guerrilla forces that eventually came to power in South Sudan, but were left on the north's side of the border after the south became independent in July 2011. The separation followed a peaceful independence vote guaranteed in a 2005 peace deal that ended decades of civil war.

Since the fighting in South Kordofan began more than a year ago, access to the remote region by the United Nations and international aid agencies has been restricted by the Sudanese government, making it difficult to verify conditions in the area.

The Enough Project said upcoming harvests will be lower than normal because many people in Nuba have not been planting crops because of the threat of bombardment by Sudanese war planes flying overhead. The aid group's survey found that 65 percent of households in Nuba have less than one week of food in stock.

"This is particularly troubling because food is not readily available for purchase, the harvest is low-yielding, and incomes are scarce or non-existent," the Enough Project said.