MILTON, N.Y. (AP) _ Firefighters worked on a ''wet and smother'' assault Monday against a tenacious underground landfill fire that forced the evacuation of more than 25 families.

A contractor continued digging large trenches on two sides of the smoldering area Monday while acrid smoke poured from about two acres of the 17-acre landfill.

When finished, the 30 feet wide by 30 feet deep and 400 feet long trenches are to be filled in with dirt to form a firebreak similar to ones used in fighting forest fires. Emergency services officials have used such phrases as ''supply lines'' and ''establish a perimeter'' to describe tactics used against the blaze.

''It sounds like the military with all these terms,'' said Paul Lent, director of emergency services for Saratoga County.

The fire, which started Thursday night, forced the evacuation of 25 to 30 families. Most of the people had found lodging with friends or relatives and are still being urged to stay away from their homes, Lent said.

Saratoga Hospital reported 35 people, most of them firefighters, had been treated for respiratory problems from the smoke.

Lent said more than a million gallons of water and 30,000 cubic yards of dirt would be used in the county's ''wet and smother'' strategy for putting out the fire.

''But we have to make sure it's out before we bury it or else we'll be back - and soon,'' Lent said.

Lent said it would be premature to estimate when the fire would be completely out.

''The fire chief feels he's gained on it today, but there's still a lot of fire out there,'' said Lee Palmer, spokesman for the Saratoga County emergency services coordinator.

Traces of toxic polychlorinated byphenyls, or PCBs, were found in the smoke but the state Department of Health said the levels of the chemicals did not exceed federal standards. More air and water tests were conducted Monday by the health department and the state Department of Environmental Conservation. Results from those tests were expected to be released Tuesday, Lent said.

The city of Saratoga Springs kept in effect a health alert, urging residents of its west side to stay inside if they have respiratory problems. Sunday's overcast skies and westerly winds helped spread the smoke over the city, but by Monday it had cleared, although a slight acrid smell remained.

Lent said the dump contains plastic piping, gypsum board, wood and other building materials placed there by its owner, John Rock, who has been charged with operating a dump without a permit.

Rock's landfill is classified by the state Department of Environmental Conservation as a construction and demolition debris landfill. Until this year, permits for such landfills were not required if the dumps were to be in operation for less than a year.

''The concern is that a large number of them are not even reported to us,'' said Bob Phaneuf, associate sanitary engineer for the DEC's Division of Solid Waste.

He said construction debris leaves large underground air pockets that allow fire to spread more easily than it would in a municipal landfill.