Kommunal and the Swedish Bus and Coach Employers' Association (BuA) have failed to reach an agreement, which means several more cities will likely be affected by the strike from midnight Monday.
"We have not been in touch with the employers and plan to extend the strike tomorrow," Kommunal spokesperson Carin Larsson told Swedish Television News on Sunday.
BuA has no plans to initiate talks with Kommunal before the end of the Midsummer weekend.
"They are the ones who started the strike so they should contact us," said BuA CEO Peter Jeppsson. He added that the union has shown no "willingness to compromise".
Parts of bus traffic in Strängnäs, Halmstad and Västerås/Hallstahammar will come to a complete standstill Monday and in Norrköping some bus routes and trams will be taken out of service.
Garage and service staff in Jönköping, Gävle and Östersund will also walk out, which may lead to further bus delays.
If a deal is not struck by the end of the week, the strike will be further intensified on Saturday June 29th. Kommunal has warned it will then pull another 1,400 employees.
That would bring large parts of Stockholm's bus traffic to a standstill, including airport coaches.
In addition to wages, Kommunal has said the key issues for their members are the length of breaks, the ability to rest for a full day and protection from being sacked if a new bus company takes over bus traffic operations.