Ethnic Somali rebels in the Ogaden have significantly intensified their secessionist war, ambushing Ethiopian supply convoys and restricting government troops and their Cuban supporters to the region's major towns.

The increased fighting in recent months, combined with Ethiopian retaliation against villages offering shelter to the irregulars, has generated a gigantic stream of refugees into Somalia filling 25 camps and straining the government's ability to keep them alive.

The spring offensive by the Western Somalia Liberation Front, which is supported by the Somali government in Mogadishu, also has led to a dozen Ethiopian bombing raids as far as 20 miles inside Somalia and dozens more against prorebel villages within the 127,000-square-mile region known as the Ogaden.

It has brought fighting to its highest level since the withdrawal of regular Somali troops in March 1978 after a crushing defeat by Ethiopians backed by Soviets and Cubans equipped with sophisticated Soviet arms. The debacle led to a quiescent period during which the guerrillas regrouped and rearmed, and the world largely forgot about their determination to secede from Ethiopia under Somali patronage.

Somalia claims that the southern desert region of Ethiopia known as the Ogaden rightfully should be part of Somalia because most of its inhabitants are ethnic Somalis. Somalia asserts that the Ogaden's inclusion within Ethiopia's internationally recognized borders as colonialist legacy.

The Soviet Union and Cuba intervened in the conflict when it appeared that the rebels, backed by regular Somali forces, would succeed in wrestling the Ogaden away and possibly also topple Ethiopia's Marxist government, which was already preoccupied with a revolt by secessionist Eritrean guerrillas.

The result added new complications to one of Africa's longest running problems. The Soviet Union lost its strategic base at the Somali port of Berbera on the Horn of Africa, and the United States has had to carefully balance a determination to stay out of the conflict with a desire to cement ties with Somalia and acquire a naval base there.

According to Western diplomatic sources, about 12,000 of the 13,000 Cuban troops in Ethiopia are currently deployed in the Ogaden, along with a few hundred of the 1,000 to 2,000 Soviet advisers in the country. There is Ethiopia's approximately 200,000 troops are currently in the disputed region.

Despite the Soviet and Cuban intervention, however, the Ethiopians never captured control of the countryside and the quiet period apparently was ended. A two-day tour of rebel-held areas near the Ethiopian town of Jigjiga in the northern Ogaden showed total guerrila control of a broad stretch of the scrub-brush countryside between the northern Somali border in the Hargeysa region and Jigjiga's outer defense.

The rebels were dressed in ragged civilian clothes, sometimes traditional long skirts, or captured Ethiopian uniforms. But they were heavily armed with AK47 assault rifles, the ubiquitous Soviet-made commando weapon, in addition to light Soviet-designed machine guns and RPG2 and RPG7 shoulder-fired, rocket-propelled grenade launchers.

Other independent observers who made a similar tour of the central Ogaden said they drove in Land Rovers for miles across the flat terrain without seeing a challenge to the guerrillas' freedom of movement on back roads and in the small farming villages that dot the countryside.

The independent eyewitness account showed Ethiopian forces able to defend only the population centers such as Jigjiga, Harar and Dire Dawa in the north, and Ginir and Imi in the center, Rebel officers said these towns are garrisoned with Cuban troops and Soviet advisers, backed by Soviet-supplied tanks.

Guerrilla leaders, whose accounts are supported by diplomats in Somalia, say their forces can often move near the edges of even these towns as long as Ethiopian Mig or F5 fighters do not attack. A May Day speech to military cadets in Harar by the Ethiopian leader, Col. Mengistu Haile Mariam, was punctuated by the sound of nearby battles on two sides of town rebel officials said.

Resupply convoys on main roads, even protected by T62 and T55 tanks, frequently are ambushed between towns, the rebels say, with heavy losses in trucks and arms. Five attacks were reported in the last week on the road linking Harar and Jigjiga. One convoy also was ambushed late last week on the road leading out of Jigjiga to the town of Capri-Payah in an operation that guerrilla leaders said was typical of their tactics.

Abdi Ahmed, a shaven-headed 21-year-old youth from Jigjiga, displayed a wounded hand he said he suffered in fighting and described the rebel harassment. Several Western Somali Liberation Front units of a dozen men each lay in wait until most of the convoy had passed, he said, then opened up with RPG7s and machine guns to cut off the tail end along with protective armor bringing up the rear.

The fighting, which isolated a small group of trucks from the main convoy, lasted desultorily for 24 hours, he said. It resulted in the destruction of two Ethiopian tanks and five trucks along with the capture of more than 200 rifles at a cost of two guerrillas killed and eight injured, his officer claimed.

Although most of the convoy went on, the rebels said, it would be attacked farther down the road several times in similar harassment raids, chipping at the supplies and attempting to isolate the defending armor. These attacks are designed to reduce Ethiopian military presence in towns by creating supply shortages and to discourage Ethiopian sallies into the countryside that the rebels call their own.

The degree to which it indeed belongs to the guerrillas was illustrated by a two-mile walk in broad daylight from this rebel outpost 20 miles northeast of Jigjiga to a loaf-shaped Ethiopian-held hill called Subul Udelay. A column of about 20 rebels and three journalists marched single file in full view of Ethiopian guns to within eyesight of the Ethiopian positions at the base of Subul Udelay.

"They won't shoot," said a smiling liberation front official, Suleiman Harsan. "They know if they do, they will have trouble. They don't want any trouble."

The unit commander, a slim youth wearing thick-soled clog shoes and carrying a late-model AK47 Kalashnikov with a collapsible stock, ordered a pullback, however, when he noticed unusual Ethiopian movement on the hillside. He feared a patrol by an Ethiopian armored personnel carrier, Hassan explained.

Hundreds of guerrillas, some barefoot and barely into their teens, squatted behind clumps of brush across the flatland leading to the hill. Many carried RPG7 grenade launchers capable of disabling a tank. They were assisted by others carrying AK47s and the apparently plentiful tubular green rockets that propel the conical RPG grenades.

Hassan said much of the ammunition and weaponry comes from ambushed Ethiouian units. Other sources said much of it also comes from the Somali Army, which was supplied by the Soviet Union in the early 1970s and backs the guerrillas with training, transport and other assistance. Aid also comes from Iraq, Syria, Kuwait and Algeria, according to Ahmed Hussein Heile, a member of the 16-man Western Somali Liberation Front executive committee in Mogadishu.

Guerrilla leaders decline to reveal the number of their nafti ghureal, the Somali equivalent of Palestinians' fedayeen, operating inside the Ogaden. Independent sources estimate the irregulars number between 20,000 and 30,000. Their leaders claim they control about 70 percent of the countryside, more at night.

The northern Ogaden Liberation Front commander, Sherif Hussein, whose headquarters are in the northern Somali town of Hargeysa, said guerrilla officers were trained by Iraquis, Palestinians, North Koreans and even Cubans before the Soviet Union switched sides in the Horn of Africa in 1974. He himself received training in infantry tactics and explosives in North Korea, Hussein said.

Stocks of antitank and antipersonel mines were stored in tents at liberation front outposts in the Ogaden, and many of the region's roads are mined. But Hussein said his irregulars need mortars and mobile artillery to match the firepower of Ethiopian troops, along with shoulder-fired SAM7 antiaircraft missiles to defend against attacks by low-flying aircraft. s

"We are expecting a long war," he said. "That's what we are planning for. If there were no foreign Cuban and Soviet intervention, it would be much shorter."