TORONTO, Canada (AP) _ The two sides in Ontario's 15-day-old beer dispute resumed negotiations today after a week of separate discussions with Labor Ministry mediators.

Victor Pathe, an assistant deputy labor minister, said progress was made during the so-called ''exploratory talks'' between Ontario's three largest brewing companies and unions representing about 4,000 workers locked out since Feb. 26.

Pathe said the two sides discussed local issues today and added that it was ''impossible'' to predict if a breakthrough would ensue.

Beer supplies across the province have been drying up since employees represented by the Brewery Workers Union and the National Union of Provincial Government Employees were locked out at plants owned by Molson's Ontario Breweries, Labatt's Ontario Breweries and Carling-O'Keefe Breweries of Canada Ltd.

Union workers were also locked out of 450 retail outlets of Brewers Warehousing Co. Ltd.

Union spokesmen say the main issue is job security. Union workers fear the growing popularity of aluminum cans will lead to layoffs of bottle handlers and cleaners.

Since the shutdown, Ontario's 11,000 licensed outlets have seen beer stocks and business dwindle despite government measures to import beer from the United States and other provinces.

Citizens have swarmed to the few Ontario breweries not involved in the labor dispute.

Police have charged some people with illegally carrying beer from neighboring provinces into Ontario.