The Soviet Union and Cuba have more than doubled their military personnel in Ethiopia in the last three weeks in an apparent prelude to an Ethiopian counteroffensive against Somali forces in the embattled Ogaden region, U.S. sources disclosed yesterday.

The State Department voiced concern yesterday over a "substantial increase" in the number of Russian and Cuban military advisers that has accompanied a massive air-and-sea lift of Soveit arms to Ethiopia.

A spokesman declined to give precise estimates on the influx of foreign military except to note that the department had said earlier that there were about 400 Cuban and 100 Soviet military advisers in Ethiopia a month ago.

Privately, officials said that more than 1,000 Soviet and Cuban military personnel are currently in Ethiopia and that some of them are believed to be with the Ethiopian troops in the Ogaden area, now largely in Somali hands.

The influx of arms and personnel appears to reflect Moscow's determination to shore up the embattled leftist Ethiopian government.

The Soviet effort in Ethiopia has become a major issue in the current round of Soviet-American negotiations on limiting arms sales to third countries, when began here this week.

Earlier this week, State Department officials accused the Soviet Union of flying without permission over several countries in an effort to covertly conduct its airlift of weapons.

The Soviet airlift, which started three weeks ago, reportedly involves unauthorized flights over a number of countries including Egypt, Pakistan, Turkey and Yugoslavia. It supplements sea shipments of heavy equipment and also involves deliveries of Soviet-built Mig jets.

The Soviets are apparently pulling out all stops to resupply Ethiopia, replacing the long-time U.S. role as the key arms supplier. Estimates of Soviet arms supplies this year range as high as $800 million.

Apart from facing unrest and dissension in Addis Ababa, the three-year-old regime of Lt. Col. Mengistu Haile Mariam has been beset by military setbacks both in Eritrea in the north and the Ogaden in the south-east.

A major battle was still under way for the control of the Red Sea port of Massawa, the main link supply route for Asmara, the Eritrean capital. Eritrean rebels claimed yesterday that they have captured all approaches to the city in the assault on the Ethiopian naval base there. Ethiopia said, however, that no part of Massawa has fallen to secessionist forces.

The Somalis are said to control about 90 per cent of the Ogaden area while the Ethiopians still control the region's two major towns, Harrar and Dire Dawa.

Meanwhile, there were fresh outbursts of political violence in Adis Ababa in which up to 50 persons were reported killed overnight. Two U.S. congressman, who ended a visit to the Ethiopian capital yesterday, were quoted as saying that they had seen bodies strewn in Adis Ababa with notes pinned on their back identifying them as "counter-revolutionaries."

"There is no doubt that there is a reign of terror in Adis Ababa today," Rep. Don Bonker (D-Wash.) said. "There is an air of suspicion and fear everywhere as a result of the almost nightly killer."

Bonker and Rep. Paul Tsongas (D-Mass.) have toured Somalia, Ethiopia and Egypt and are scheduled to present their findings to Vice President Mondale.

The Ethiopian strongman received the two congressmen earlier this week, the first such act since he assumed power last year.

The United States has refused to supply arms to both Ethiopia and Somalia although last summer Washington entertained the idea of sending weapons to Somalia when its government turned against Moscow. the Ethiopians had earlier renounced their defense agreement with the United States and turned to Moscow for help.

If the current Soviet buildup in Ethiopia leads to a major Ethiopian push to regain control of the Ogaden, the United States may be forced to reconsider its hands-off attitude toward the Horn of Africa.

Moreover, the Somalia-Ethiopian conflict would disrupt current efforts by the two superpowers to bring an element of stability in international arms sales as well as establish an equilibrium of forces in the Indian ocean.

In the arms sales talks here, the world's two principal arms merchants are in effect trying to determine whether they can reach a set of rules on supplying arms to Third World countries.

On a different plane, the United States and the Soviet Union have reached a tentative agreement to stabilize competition in the Indian Ocean. But Somalia's expulsion of the Soviets last month has deprived Moscow of a naval facility at Berbera and tipped the balance in favor of the United States, which maintains a base at Diego Garcia.

In recent weeks the United States has stepped up criticism of Cuba for increasing troops in Angola and has voiced concern over Cuban involvement elsewhere in Africa.

Somalia has claimed for some troops in Ethiopia and that the Soviets were airlifting arms, but U.S. officials generally doubted such reports.

"Lately, some of the Somali claims seem to be becoming self-fulfilling prophecies," a U.S. source said.