NAIROBI — 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 US advocacy group said Thursday, citing research collected from a region where aid groups are not 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.’’