Dozens of Soviet and East European cargo ships, protected by Soviet navy armed escorts, are taking heavy combat equipment to Ethiopian ports south of Massawa, Western intelligence sources said tonight.

They said more ships were involved in the operation that when the Soviet Union sent weapons to Angola during the last days of the civil war there.

The ships were unloading heavy tanks, lighter armored vehicles, rockets, heavy artillery guns and ammunition at the ports.

They said the ships, coming from Black Sea ports, were passing through the Suez Canal on their way to Ethiopia, where Eritrean guerrillas have 6,000 Ethiopian troops trapped on the waterfront of Massawa.

Ships of the American 6th Fleet were shadowing the Russian ships in the Mediterranean while U.S. 7th Fleet ships were watching them from off Ethiopia.

American spy satellites tas well as electronic intelligence aircraft are keeping close watch over the situation, the sources said.

More than 225 Soviet air force cargo planes - some 15 per of the Soviet Union's transport fleet - have also been flying arms, equipment and personnel into Ethiopia, the sources said. Previous reports had put the number at 50.

The planes are mainly big Antonov-22 long-range, heavy-duty transports but also include Tupolev 76 planes, the heaviest in the Soviet inventory.

Maximum payload of the Antonov-22 is 80 tons though this would be reduced considerably by the lenght of the flights involved, the sources said.

They said the airlift to Ethiopia was comparable with the operation to supply arms and equipment to Soviet-backed forces in the Angola war.

The Russian planes are said to use several routes to get to Ethiopia with some landing at Aden and offloading equipment for sea transport to Ethiopia.

The planes mainly fly from Soviet Black Sea airports but some have been leaving from airports in Yugoslavia and Bulgaria, the sources said.

They said the Soviet Union has closed some railroads to civilian traffic in southern Russia and are using them exclusively to ship arms and equipment to seaports and airfields.

The Russians opened up their weapons stockpiles behind their front-line divisions along the Chinese border to give aid to the Ethiopians, according to the sources. War stocks were taken from Tashkent and Alma Ata and flown to Aden by a route taking the airplanes along the Gulf of Oman.

Other cargo planes took weapons across the Arabian sea and the Indian Ocean to Maputo, the capital of Mozambique, presumably for transhipment to Ethiopia, the sources said.

The Soviet Union is said to have launched a new military spy satellite, which was believed to be playing a key role in controlling the complex operation.

The sources said the Soviet airlift to Ethiopia really got under way on the night of Nov. 26 - when American spy satellites and surveillance ships of the 6th and 7th fleets noticed increased Soviet air traffic.