When their convoy arrived this morning at a shabby refugee camp in southern Burundi, the U.N. officials climbed out of their vehicles and greeted uniformed men visible in a house near the gate. The camp was reserved for civilians forced out of their homes by Hutu rebels who have been fighting Burundi's Tutsi-led army for most of the decade in a civil war whose estimated 200,000 victims have been almost all civilians.

The U.N. team had come, escorted by the army, to learn what food and medicine the people needed. But when the uniformed men stepped out of the house, according to an account pieced together from survivors, the U.N. officials realized they were impostors and the camp was controlled by the rebels. Armed men in civilian clothes herded the eight foreign nationals against a wall, stole their valuables and, after hearing that they were aid officials, began to walk away.

But, according to the survivors' accounts, one rebel turned back and asked aloud: Why should these people be allowed to live? At point blank range, he shot Luis Manuel Zuniga, 53, in the head. The Chilean, who ran UNICEF operations in Burundi, fell dead.

The rebel then killed Saskia Von Meijenfeldt, the 34-year-old Dutch woman whose glasses had been stolen from her minutes earlier, then returned. She was a logistics specialist for the World Food Program, the U.N. agency that feeds the hungry.

"These people were executed," said World Food Program spokeswoman Michele Quintaglie, who reconstructed the shootings from accounts of the six U.N. workers who survived. They lived, Quintaglie said, by fleeing into the countryside after a U.N. security officer interceded with the gunman.

In the gunfire that exploded around them, seven others died, all Burundians.

The incident threw into stark relief the deteriorating security situation in Burundi. The nation of 6 million has been calm for most of the year as the rebels and the army took their fight to neighboring Congo (formerly Zaire). But about a month ago the guerrilla war resumed in Burundi's countryside and even in the capital, Bujumbura.

The government, which is dominated by an elite sliver of the minority Tutsi population, has responded by moving large numbers of Hutus off their land while the army sweeps the countryside. More than a quarter of a million people have been confined to "regrouping" camps. Conditions are said to be wretched, and international agencies have pressed for relief.

"The rebels want to make the international community believe that these are not protection sites but concentration camps, as they call them," said Appolinaire Gahungu, spokesman for President Pierre Buyoya. "This maybe is a message sent to humanitarian organizations to keep from helping."

But it was unclear whether the attack was planned. Coming less than a month after a UNICEF doctor was murdered in Somalia and a day after a U.N. worker was killed in Kosovo, the deaths underscored the risks of relief field work, especially on this continent torn by wars where discipline seldom figures.

Last year, for the first time in its history, the United Nations had more civilian workers than peacekeepers killed in the line of duty. Of the 50 World Food Program workers killed in the past 11 years, half were murdered. Von Meijenfeldt's death brings the agency's 1999 death toll to an even dozen.

"This is the scary thing about being an aid worker," said Quintaglie. "You're at the whim of one guy with a gun. It's the totally unpredictable."