Ethiopian forces opposed to the military government in Addis Ababa have reportedly captured three towns in northern and western Ethiopia in the past three weeks and are now theatening several others in what appears to be a major escalation of fighting in the civil war there.

A leader of the opposition Ethiopian Democratic Union, Gen. Iyassu Mengesha, said here today that 122 Ethiopian soldiers and policemen were killed and more than 200 surrendered in a battle for the town of Humera in Ethiopia's Begemdir Province early this week. The entire army garrison of 1970 troops surrendered to opposition forces Sunday and the town is now under their control, he said.

THe Ethiopian government has not confirmed the fall of Humera, a market town with a population of 25,000, but it has announced the suspension of all flights to the airfield there.

Meanwhile, it was reported that the separatist Eritrean Liberation Front had overrun two towns in Eritrea Province.

The Democratic Union leader said that none of the towns captured or threatened by his forces had much strategic value, but that their fall might have a serious effect on the Ethiopian army itself.

"It's mainly psychological but their capture can affect the whole area politically as well," he said.

The Ethiopian Democratic Union, a front of various groups opposed to the military government in Addis Ababa, now has about 6,000 troops fighting an undetermined number of Ethiopian soldiers in Begemdir Province alone, according to Gen. Iyassu.

Unlike the Eritrean Liberation Front, which seeks only the independence of Ethiopia's strategic Red Sea province, the Democratic Union wants to overthrow the military government.

The military government regards the union as a "reactionary" group made up mainly of dispossessed landlords seeking to restore the old imperial order and scrap its socialist revolution.

Whatever its aims, the Democratic Union appears to be gaining ground at least in Begemdir Province and posing a growing threat to the government which is already facing a deteriorating situation in Eritrea as well as armed Somali-baked opposition movements in the south and east.

The fall of Humera to opposition forces was followed by an announcement here yesterday that the nearby town of Um Hager in neighboring Eritrea Province had been overrun by the Eritrean Liberation Front, the group leading the 14-year-old struggle for the province's independence from the rest of Ethiopia.

Two Ethiopian soldiers who crossed into the Sudan were quoted by the Sudan National News Agency as saying that their company in Um Hager had been "totally destroyed" by the Eritrean Front. Other sources here said they thought most of the company's 150 to 170 troops had withdrawn north to Tessenay.

Earlier this month, the Eritrean Liberation Front overran another Ethiopian army garrison in Gerora, in far northern Eritrea Province on the Sudanese border.

In addition to the three towns that and the Eritrean Front, three others are surrounded and are expected to fall shortly, according to Ethiopian opposition sources here.

Gen. Iyassu, who said he had just returned from Humera, reported that negotiations were under way for the peaceful surrender of company-size Ethiopian army units stationed in Abdirafi and Chilga in Begemdir Province.

The ohter town that is reportedly on the verge of falling is Naqfa in northern Eritrea Province. It has been surrounded by Eritrean Front guerrillas for more than a year and is being supplied by air but with increasing difficulty.

Gen. Iyassu of the Democratic Union said an Ethiopian garrison of about 500 soldiers in the border town Metemma was surrounded by rebel forces. He said it was not expected to fall soon, because several hundred reinforcements have been sent in.

[Agence France-Presse reported from Addis Ababa that more than 10,000 people turned out in Metemma for a demonstration supporting the military government. The report said they were protesting the "betrayal" by an officer who went over to Democratic Union forces attacking Humera.The Defense Ministry in Addis Ababa announced Wednesday that the officer had surrendered to government forces.]