The possibility of a BART strike is looming larger as the transit system board considers imposing new contract terms on employees if their unions fail to agree to management's final contract offer.

BART has given its unions until the end of Thursday to respond to the district's last and final offer for a four-year contract. If the unions reject the offer, the BART board is prepared to call a special meeting as early as Saturday to move toward unilateral imposition of a new one-year contract, according to sources close to the board.

BART managers and board members are frustrated that negotiations have dragged on for months without a settlement to meet the board's goal of trimming $100 million in labor costs over four years through new work rules and higher employee contributions toward benefits.

BART officials say the district remains far apart with the Amalgamated Transit Union Local 1555, which represents train operators and station agents, while it is closer to settling with its largest union, Service Employees International Union Local 3933.

Union officials have asserted that imposing a contract would be illegal, and could lead to a lawsuit or strike, while BART managers contend the move would be legal.