Ethiopia appeared yesterday to have won a significant victory in the war-torn northern province of Eritrea by pushing through a relief column into the besieged city of Asmara.

But diplomatic observers cast doubts on claims by the official Ethiopian News Agency yesterday that government troops had broken the 10-month siege of Ethiopia's second largest city.

The relif of Asmara would be the most important success in the latest Ethiopian campaign to wrest control of the strategically located province from two groups of rebel Eritrean nationalists.

The initial success of the campaign, which began in early June, occurred Wednesday when Ethiopia announced it had taken the two key towns of Mendefera and Tessenei from the rebels.

The rebels claim control of 90 percent of the Eritrean countryside and all but five major towns. Eritrea is strategically important to Ethiopia because it contains the country's only seaports.

Ethiopia's only other outlet to the sea is through Djibouti, a tiny former French territory squeezed between Ethiopia and Somalia. The rail link from Addis Ababa to Djibouti is subject to attack by Somali guerrillas, and Somalia had made territorial claims on Djibouti as part of what it calls Greater Somaliland.

Yesterday's Ethiopian report said that a contingent of troops from within Asmara had broken through to meet another group heading north from Addis Ababa, reopening the highway between the two cities.

According to refugees fleeing to Sudan, the latest Ethiopian effort involves thousands of troops and hundreds of pieces of Soviet-supplied armored vehicles and artillery pieces.

Cuban advisers are believed to be helping the government forces, but observers say the Cubans and Soviets are not playing the major roles they did in the Ogaden region earlier this year, when Cuban pilots and ground troops were reported to be taking an active part in the fighting.

Observers say the current Ethiopian successes may reflect a strategy of retaking key Eritrean towns and leaving the countryside, to the rebels, in an effort to give Ethiopia some bargaining chips during any future negotiations on the province.

The rebels called for such negotiations a month ago, after a key Eritrean rebel leader visited Moscow amid signs of increased Soviet pressure for a negotiated settlement to the 17-year-old civil war.

The Soviets and Cubans, who have long had relations with the Eritrean People Liberation Front, reportedly balked at Ethiopian insistence that they play a major role in fighting the rebels.

Since it was besieged last fall, 60 percent of Asmara's population of 250,000, has fled and rebels have often prevented supplies from getting in even by air.

The retaking of the road between Addis Ababa and Asmara is believed to be a major goal of the current Ethiopian campaign.

Diplomatic sources said that while the relief of Asmara was a significant advance for Ethiopian forces, they doubted that the Ethiopian column that reached the besieged city had done more than break through the ring of an estimated 25,000 guerrilla troops and open the road temporarily.

"What the Ethiopians have proved is that they are back in the position they were in a year ago," one sources said. "They can at least get an armed convoy through."

The source said, however, that the guerrillas probably still control most of the road to Addis Ababa.

Yesterday's Ethiopian claims coincided with the announcement by the two main Eritrean guerrillas groups that they were withdrawing from some areas. A spokesman for the Eritrean People's Liberation Front said its unit would pull back from some stationary position because of losses suffered by other group, the Eritrean Liberation Front.