Giant Soviet Antonov transport planes have begun flying in Mig-21 jet fighters to reinforce Ethiopia in its desert war with Somalia, diplomatic sources reported yesterday from the Ethiopian capital, Addis Ababa.

The sources also said they had received reports of renewed ground fighting following a lull on the main battlefront, at the Marda Pass gateway into Ethiopia's mountain heartland.

Ethiopia's Marxist military rulers turned to the Soviet Union for help after they expelled a U.S. military assistance group in April. The Addis Ababa sources said that if Soviet technicians were there to supervise and in the air within a week. Ethiopia is reported to have sent 40 pilots and 60 mechanics to the Soviet Union recently for Mig training and is expected to get 48 Mig-21s.

The Somalia Republic, which also has Soviet arms, denies throwing regular forces into the fight against the Ethiopians but makes no secret of supporting guerrillas who are ethnically Somali, who seek self-determination for the Ogaden region and claim to have taken it from Ethiopia in nine weeks of war.

Ethiopia's remaining lifeline to the sea appeared to be in peril after saboteurs blew up a major road bridge near the Red Sea port of Assab, thereby severaing Addis Ababa from its main source of essential fuel supplies.

The attack on the Assab-Addis Ababa highway, apparently by pro-Somali guerrillas, added a grave new dimension to the war from Ethiopia's point of view, because the road carries all the fuel and much of the weaponry required and has taken on crucial importance since June, when the railway line linking with Addis Ababa with Djibouti was cut.