WASHINGTON (AP) _ Sudanese Prime Minister Sadiq Mahdi said Wednesday the specter of starvation in southern Sudan has eased with the reactivation of transportation routes that the government claimed had been sabotaged by rebel forces.

The Reagan administration had said that the lives of up to 2 million Sudanese were at risk because of the civil war, but Mahdi told a news conference that almost 150 food trucks recently were sent to urban areas where shortages were most acute.

Mahdi said efforts to reach a settlement with the rebels ceased after a mid-August incident in which the insurgents shot down a civilian airliner, killing 60 passengers.

Afterward, the government began treating the rebels as terrorists, Mahdi said.

During his U.S. visit, Mahdi has met with Vice President George Bush and Secretary of State George Shultz and had other appointments Thursday.

Mahdi said U.S.-Sudanese relations are now on a more stable basis since the ouster of the pro-Western government headed by President Gaafar Nimery in April 1985.

Whereas Sudan under Nimery was aligned to the West, the relationship is now based on American acceptance of a non-aligned Sudan and its ''born-again democracy.''

U.S. officials, who asked not to be identified, said $50 million in economic assistance for Sudan is being withheld, pending that country's adoption of a new economic plan.

The officials also said some U.S. diplomats who were withdrawn last spring because of the presence of suspected Libyan terrorists are now returning to Khartoum, the capital.

According to the officials, the number of American diplomats had been reduced to about 50 but now is being increased to slightly over 100, reflecting the improved security situation in the capital.

Mahdi said Libya has halted assistance for the rebels and the two countries now enjoy friendly relations.