SOFIA, BULGARIA, NOV. 28 -- Bulgaria's Socialist government teetered on the verge of resignation tonight after a general strike closed a crucial oil refinery and the country's largest unions announced they would join the strike on Thursday.

The Bulgarian news agency BTA reported that Prime Minister Andrei Lukanov had agreed to resign in overnight negotiations with the main opposition party, the Union of Democratic Forces, and that the Socialists and the opposition would form an interim government under a neutral prime minister. Lukanov denied this, but when pressed later on whether he would resign he replied only: "Not immediately."

The opposition trade union confederation, Podkrepa, claimed that 870,000 workers, more than 20 percent of the work force, have joined the strike. The national airline, Balkan, was grounded, with only foreign carriers permitted to use Sofia's airport. The oil refinery in the Black Sea town of Burgas closed down, as did the natural gas pipeline to Sofia. The government continued to claim that only 1 percent of the work force is on strike.

Lukanov is now under attack on several fronts. The opposition UDF has withdrawn temporarily from the national legislature, leaving his Socialist Party, the renamed communists, isolated. The opposition unions, unable to bring the country to a halt on their own, have focused their strike on the strategic areas of transportation, oil and energy. On Thursday the patchy work stoppages may become devastating as the former official union confederation calls its members out as well.

Bulgaria's Socialists were the only former communist party in Eastern Europe to win a majority in free elections. But since the June vote Lukanov has insisted that the opposition must join the government before he would implement painful economic reforms, while the UDF has held out for full authority.