Cuban Defense Minister Raul Castro is in Ethiopia to plan a counteroffensive which is expected to see Cuban troops fighting alongside Ethiopians against Somali invaders in Ethiopia's Ogaden region, U.S. government sources said yesterday.

The sources said Fidel Castro's younger brother secretly flew into the country last week to plan the military moves.

The State Department, which frequently voiced concern in the last month over the Soviet and Cuban military buildup in Ethiopia, said yesterday that "the influx of sophisticated Soviet weaponery and technicians would have the possible danger of involving Soviet or Cuban technicians in combat situations."

The statement added, however, that the United States had "no precise information" to indicate Cuban or Soviet troops "are involved in such situations."

The U.S. government sources said the younger Castro arrived in Addis Ababa, the Ethiopian capital, under such tight precautions that he did not leave his plane until it was inside a hanger, United Press International reported.

The sources, citing intelligence reports, said a Cuban artillery battalion, equipped with rockets and howitzers, had arrived in Ethiopia recently. They added that another 650 Cubans were in Harrar, the key city the Somali forces have been seeking to take in the battle for control of the contested area in southeastern Ethiopia.

Another U.S. government source estimated that it would take four to six months for Soviet and Cuban advisers to train the Ethiopians to use the new equipment, much of which is being flown in. Any Ethiopian offensive mounted sooner, therefore, would be likely to need the support of foreign troops, he said.

There have been reports that if the Soviet-Cuban buildup leads to a major Ethiopian push to regain the Ogaden, the United States may be forced to reconsider its hands-off attitude toward the Horn of Africa.

President Carter, in his press conference yesterday, criticized the Soviet buildup and said the United States had "expressed our concern to the Soviets in very strong terms." He said it was possible that the issue would be taken to the United Nations Security Council but added that the basic negotiations should take place between Ethiopia and Somalia.

Estimates on the number of Soviet and Cuban troops in Ethiopia and the value of the arms being shipped to Ethiopia have varied widely.

The sources estimated that about 2,000 Cubans, mostly combat soliders, and 1,000 Soviet advsiers are in Ethiopia, UPI reported.

Last month U.S. officials said the combined total, which had been slowly increasing since the Soviet Union and Ethiopia signed an armaments agreement in late 1976, had reached about 1,000.

The State Department said yesterday it did not have exact figures but it would "seem logical" to revise upward previous estimates.

Estimates on the value of Soviet armaments flowing into Ethiopia range up to $800 million, including jet planes artillery rockets and radar. Under the agreement Ethiopia is to become one of the few non-Communist countries to receive the MIG-23, the Soviet Union's most sophisticated figher-bomber.