The two sides held marathon talks from Wednesday afternoon until early Thursday morning in an effort to defuse the threatened walkout, but the negotiations ended without a breakthrough as the differences in positions is too wide, officials said.
The union is demanding a 9.5 percent pay raise while the management wants to freeze wages. Both sides are expected to meet again later Thursday in a last-ditch effort to reach a compromise that will keep Seoul's buses running.
The union has threatened its members will strike from 4 a.m. Friday.
The Seoul city government has put together an emergency public transportation plan, including putting more subway trains into operation, in the event the threatened walkout takes place.
Union members endorsed the strike plan on May 15 with 91.4 percent support.