Widespread and bitter fighting between Ethiopian forces and Somali-backed secessionist guerrillas for control of Ethiopia's barren eastern Ogaden region was reported yesterday.

Diplomatic sources in Addis Ababa said the Ethiopians were apparently more than holding their own.

The official Ethiopian News Agency reported that Ethiopian troops and militia have killed "thousands of inflitrating Somali soldiers and captured hundred of others" along with large quantities of weapons, including American M-16 rifles.

Attacking the U.S. and Somali governments, the agency said the capture of modern American automatic weapons "clearly shows that American imperialism on the basis of its secret deal with the Mogadishu regime, has begun indirectly to arm Aomalia to violate the territorial integrity of Ethiopia and reverse the course of its popular revolution."

In Washington, a State Department spokesman denied that the United States had provided arms to Somalia. The Soviet Union earlier this year replaced the United States as Ethiopia's main foreign ally and arms supplier when the Ethiopians expelled American military advisers and other official U.S. representatives in the country.

The Ethiopian News Agency also charged Arab governments with complicity in aiding the rebels.

Since Ethiopian leader Lt. Col. Mengistu Haile Mariam consolidated his power earlier this year, Ethiopia increasingly has accused the United States of fostering a series of plots and maneuvers to overthrow the Marxist regime.

As relations deteriorated and Washington cut off military aid to Ethiopia, Addis Ababa expelled some 400 American diplomatic and military staff members from the country.

The Ethiopian governmnet announced yesterday that the first batch of Cuban medical experts had arrived in Addis Ababa, reflecting Havana's growing commitment to Marxist Ethiopia.

Cuban President Fidel Castro announced yesterday in Havana that Cuba plans to send more than 300 doctors and paramedical personnel to Ethiopia.

More than 50 Cuban military experts have been in Ethiopia for months training the so-called peasant army - Reportedly 300,000 strong - in a tent city near the capital.

In the Ogaden Desert fighting, Ethiopia consistently has charged that Somali regulars were taking part in the fighting.

Somalia has denied the charges, saying only Somali backed irregulars fighting for the Western Somalia Liberation Front were involved.

The Ethiopian News Agency reported that Kenya has rejected a request by Somalia that its troops be allowed to cross through the northeastern corner of Kenya en route to Ethiopia.

The agency said that Kenyan government rejected the request as a violation of the charter of the Organization of African Unity and the Arusha Memorandum of 1967 by which Somalia and Kenya agreed to end a five-year-old state of emergency.

Somalia claims the Ogaden as its own territory and gives arms and other support to seccessionist guerrillas of the Western Somali Liberation Front fighting in the area.

There are fears among diplomatic observers in both Ethiopia and Somalia that the two neighboring states could be drawn into a full-scale conflict over the Ogaden dispute.

The Ethiopian News Agency report indicated that the fighting was concentrated around the towns of Gode, Kebri Dehar and Warder in the southern Ogaden and Dire Dawe and Harrar in the north.

A number of vehicles with Somali number places allegedly used for "transporting arms and Somali soldiers into Ethiopia" were captured in recent fighting, the agency said. Hundreds of hand grenades, heavy machine guns, antiaircraft guns, 60 mortars and machine guns also were captured, it said.

According to unconfirmed reports, the Ethiopians have managed to recapture the southern town of Gode from front guerrillas who took it earlier this month.

Monday might Mogadishu Radio in Somalia broadcast a communique from the Western Somalia Liberation Front saying 750 Ethiopian troops were killed and 10 Ethiopian aircraft destroyed in the fighting outside Dire Dawa.

Diplomats reached by telephone in Addis Ababa said Ethiopia appeared to have launched a major offensive to drive out Somali-bakced insurgents who blew up bridges on Ethiopia's only railway to the port of Djibouti in early June and cut the mountain roads linking Dire Dawa, Harrar and Jijiga.

The diplomatic observers said the big government drive appeared to be the first major test for the People's Army.

The Ethiopian News Agency said army and militia reinforcements sent to Dire Dawa, the main town on the railway between Addis Ababa and Djibouti, were greeted by thousands of people lining the streets.

The people of the town, it said, "are now busy providing the men in uniform and members of the People's Militia with provisions."

It said a number of unidentified "Arab nationals" were fighting with the insurgents. Two of these Arabs were killed and one, a doctor captured.

In addition, the Ethiopians had captured dried dates packed in Iraq, the report said.

"This shows clearly that reactionary Arab regimes and their patron, international imperialism, are bent on endangering the unity of Ethiopia and at reversing its revolution," the agency declared.

Diplomatic sources said Addis Ababa has been preparing a counter-offensive for some time.

But Mogadishu, one guerrilla leader told a reporter that the rebels had launched their own offensive against the Ethiopian army.

"We are prepared to fight until the desert sand is soaked in blood," the guerrilla leader said. "The blood has begun to run."