Increased fighting between Somali insurgents and Ethiopian forces, coupled with an autumn drought, has led to a drastic upsurge in the flow of refugees into Somalia from Ethiopia's disputed Ogaden region, according to U.S. sources and U.N. officials.

These sources say the refugee population of Somalia Has swollen to over half a million. The new arrivals have been streaming across the border at the rate of up to 1,500 a day in recent months, compared to 800 to 1,000 a day in the first three quarters of last year.

Aggravation of Somalia's refugee problem comes as the United States is expressing renewed interest in the volatile Horn of Africa, especially Somalia and neighboring Kenya, in the search for military facilities to protect Middle East oil lanes.

The Carter administration has decided in principle to supply Somalia with military equipment such as trucks, transport aircraft and radar. The offer reportedly is linked to the U.S. effort to obtain air and port facilities in that country.

Washington has been reluctant to provide Somalia with military aid ever since Somali forces invaded Ethiopia's Ogaden region in the summer of 1977 in support of ethnic Somali insurgents, only to be repulsed by Cuban and Ethiopian forces. That reluctance now appears to have been eclipsed by the pressing need for military facilities to project American power into the Indian Ocean and Persian Gulf area.

To some extent, the Somali refugee problem has been overshadowed by the geopolitical maneuvering, but the half million refugees clogging Somalia's 21 camps are visible reminders of continuing conflict on the Horn of Africa.

Their plight underscores the possibility of confrontation by proxy with the Soviet Union, Ethiopia's chief arms supplier, if the United States establishes military ties with Somalia.

U.S. officials believe that a shortage of rain in the Ogaden's brief September-October rainy season may have limited forage for the nomads' livestock, thus aggravating the refugee flow as men stayed behind with their herds and sent their families to the camps.

Although the drought may have been a contributing factor, fighting in the Ogaden also appears to have increased gradually in the past six months and official sources said regular Somali forces have been used in the area in the last two months.

The Somalis say that the Ethiopians are deliberately trying to empty the Ogaden of its ethnic Somali civilian population, a position also argued in a speech Jan. 31 to the House by Rep. Thomas E. Petri (R-Wis.), a former Peace Corps volunteer in Somalia.

"There is much impartial evidence that the flood of refugees has been caused by the indiscriminate bombing by the Ethiopian Air Force of innocent civilians, poisoning of water holes and strafing of the nomads' livestock," he said.

The House Africa Subcommittee is to hear testimony today on the Somali refugee situation.

U.S. officials say it is difficult to gauge precisely the population base in southern and eastern Ethiopia from which the refugees, mostly ethnic Somalis and Galla tribesmen, are coming.

The most conservative estimates run to about 2 million people, but one official said "the number may actually be as high as 5 million."

Both U.N. and U.S. officials believe the population of Somalia's refugee camps could rise as high as 800,000 or a million by the end of this year if the refugees keep coming at the current rates.

If the food continues, one U.S. official observed, "there could eventually be as many refugees as there are citizens in Somalia." Somalia's population is 3.4 million.

According to figures of the United Nations High Commissioner for Refugees, the refugee population in Somalia passed the 500,000 mark in January. It is the largest refugee problem in Africa, rivaled in size worldwide only by the 500,000 to 600,000 Afghan refugees in Pakistan and the nearly 400,000 Cambodians along the Thai-Cambodian frontier.

The Somali government estimates that, in addition to the half million refugees in the camps, another 700,000 are roaming the countryside, which would put the real total at 1.2 million.

Senior liaison officer Chris Thorne of the U.N. High Commissioner for Refugees' Washington office said that tribesmen flocked into Somalia from the Ogaden at an average rate of a thousand a day in 1979. In the last quarter of the year, however, this rate surged to 1,500 a day.

About 90 percent of the camp populations are children under the age of 15 and women, according to U.N. figures.

In October, the Somali government appealed for $71 million in relief aid.

Dr. Kevin M. Cahill, a New York physician who visited the Somali camps last fall, recently wrote that "infectious diseases -- malaria, tuberculosis, hepatitis, dysentery, bronchitis -- are rife and the potential for truly decimating epidemics of cholera, for example, is frightening predictable."

He added, "The death rate is astronomic. In one camp of 41,000 women and children, there had been 2,000 deaths in the last two weeks, with 41 pregnant women have died from dysentery during the week I visited the camp."