Kenyan warplanes intercepted an Egyptian cargo jet over Kenya yesterday and forced it to land at Nairobi airport, where tons of military supplies bound for Somalia were found aboard the plane, the official Kenya news agency reported.

A Kenya security official said that the Egyptair Boeing 707 carried more than 200 boxes of ammunition that included 122-mm, artillery shells. The official said the seven-member crew was under arrest.

It was the fourth Egyptian plane this week to have overflown Kenya en route to Somalia, the Kenya news agency said. The agency said an Egyptian request for overflight permission had been turned. Kenya, which has a territorial dispute with Somalia, has been supporting Ethiopia in its war with Somalia in the Ogaden region.

Somalia, meanwhile, appealed to black African states to end their silence about Soviet and Cuban intervention in the fighting, Washington Post correspondent David Ottaway reported from Lusaka, Zambia.

Somalia's ambassador to Zambia said, "I cannot believe African countries approve that foreigners from outside Africa should come in the thousands . . . to kill Africans by the thousands."

In Mogadishu, Somalia's capital, military analysts said that the Ethiopian counteroffensive in the Ogaden had slowed to a virtual standstill. They said the Ethiopians and their Soviet military advisers are preparing to bypass entrenched Somali positions and drive to the border.