Sudanese rebel leader John Garang has denied a U.N. request to allow a famine-relief convoy into rebel-held southern Sudan where, according to U.N. estimates, 1 million people are threatened by famine.

Winston R. Prattley, senior U.N. official in Sudan for famine relief, said the Sudanese People's Liberation Army denied his personal plea to allow safe passage of food on the ground that it would interfere with their "strategy to gain control of southern Sudan."

By U.N. estimates, 1 million people are "at risk of starvation" in southern Sudan as a result of continuing civil war, tribal fighting, drought, shortages of seeds and a severe infestation of locusts.

Prattley flew secretly last weekend to Addis Ababa, the Ethiopian capital, to meet with Garang and other leaders of the Sudanese People's Liberation Army, which receives arms and financial support from the Ethiopian government.

Western diplomats here say that the force, with between 15,000 and 20,000 men in arms, controls about one-third of Sudan and half its 20 million people. The rebellion is rooted in differences between the predominantly Moslem north and the Christian and animist south.

Prattley said he asked that Garang's forces allow a convoy of 60 trucks to move relief food north from the Kenyan border along a 310-mile road to Juba, the major city in southern Sudan.

"They said they were fully aware of the problems affecting people in the south but they said no to my convoy," Prattley said.

Sudan has a record grain surplus in the eastern part of the country, relief officials say, but it cannot move south because of the threat of rebel attack. Alternate supply routes from Uganda are blocked because of civil war there, relief officials said, and roads from Kenya are not safe for trucks until they are cleared of land mines placed by Garang's forces.

The one prospect for food aid in the south appears to be World Vision, a private American relief agency. According to spokesman Steve Reynolds, World Vision has decided to try to move its American-donated food south from central Sudan using private Sudanese trucking contractors.

Reynolds said the approach of the rainy season in May, which will halt land transport of food for several months, is forcing World Vision "to go on its own without the U.N."

According to western diplomats here, Garang's force, since November, has begun what appears to be a new strategy to seize towns and cities in the south. They took Yirol, about 150 miles north of Juba, in late December and appear likely to take nearby Rumbek.

Diplomats here said that if Garang allowed large amounts of relief food into southern Sudan it would undo his strategy of isolating and seizing population centers. The rebels have told some diplomats here that they will soon launch an offensive to take major southern towns, such as Wau and Juba.