The Teamsters union reached a tentative agreement with Giant Food on a new contract late yesterday, but negotiations were continuing with Safeway to avert a possible strike by truck drivers that would affect food deliveries to more than 150 Safeway supermarkets from Baltimore to Fredericksburg, Va.

The contract disputes centered on company plans to cut the pay for newly hired drivers and set up separate "two-tier" pay scales for veterans and new drivers, according to company and union officials.

"We have a tentative settlement that the union will vote on Sunday," said Charles Scott of the Federal Mediation and Conciliation Service, after shuttling back and forth between rooms full of company and union officials at the mediation headquarters in Northwest Washington. Scott said he could not divulge details of the Giant settlement, and he said "problems remain" in the Safeway talks.

Late last night Scott said Safeway and the union were "still negotiating," and that he was "hopeful we'll be able to settle" on a new contract. He could not say when agreement might be reached, however, predicting, "We may be here all night."

The Teamsters, who have threatened a strike by 500 drivers if the companies persist in demanding a lower pay scale for new drivers, would attempt to shut down both supermarket chains' huge warehouses in Landover, but they probably would not picket the affected stores, according to a source familiar with the union's strategy.

"Both sides are playing hardball on this issue. They both have indicated they are willing to take a strike," said a spokesman for the United Food and Commercial Workers union, which represents more than 10,000 supermarket employes who would be affected by a walkout.

Both firms are eager to reduce their $14-per-hour wage costs and are strongly committed to the so-called "two-tier" pay scale, company officials have said. Safeway has begun training management personnel to drive 18-wheel rigs in the event of a strike, said a reliable source. Company officials could not be reached for comment.

Veteran truck drivers earn $13.87 per hour at Giant and $14 at Safeway, and both firms have complained in bargaining sessions that competing companies -- including some with Teamster contracts -- pay far lower wages. Peoples Drug Stores pays its Teamster drivers $9.50 hourly, and Richfood, a Richmond distributor, pays $8.26 an hour, according to Giant.

The companies have proposed reducing pay for new drivers to roughly $8 an hour, with a maximum of about $10, although a compromise proposal made by Giant yesterday would increase the pay range to roughly $9.80 to $12.50. The compromise would enable one new driver to reach the veterans' pay scale for each veteran driver who departs.

Phillip Feaster, president of Teamsters Local 639, has said the union would "fight it to the last straw" to resist the dual pay scales, but he was not available for comment on the latest company proposals. Such two-tier contracts have become increasingly popular as a means of reducing labor costs without cutting pay of current workers, but some unions have strongly condemned the idea as unfair and divisive to workers who would do the same job at different pay rates.

Giant and Safeway already have implemented two-tier systems for their grocery clerks and other employes who belong to UFCW, which unsuccessfully resisted the proposal in a contract settlement last year. Veteran supermarket clerks will earn up to $12.50 per hour under the existing contract, but new hires have a maximum of $8.55. New hires also receive lower pensions and fewer holidays.

The Teamsters last week met with the UFCW and other unions to seek assistance in the event of a strike.

The local grocery industry has not had a major strike since 1974.