More than 100 drivers in Weymouth and Bridport in Dorset, are currently involved in an eighth week of industrial action.
Bus firm First Dorset said a further walkout was "unnecessary".
The union Unite said it had contacted the conciliation service Acas, but accused First Dorset of not yet agreeing to the terms of reference.
First Dorset said it welcomed Unite's request for formal arbitration but added a date had not yet been set.
Unite has said Weymouth and Bridport drivers earned Â£8.80 an hour and it wants this to rise to Â£9.50 over three years to match the wages of drivers in Yeovil, Somerset.
But First Dorset has said the pay increase is "unaffordable".
The first round of action began on 20 June, with the current round due to end at 04.29 BST on Monday.
However, further action will now take place from 04:30 on Monday until 04:29 on Tuesday, from 04:30 on Wednesday until 04:29 on Thursday, and from 04:30 on 26 August until 04:29 on 29 August.
First Dorset said the firm was providing more than 90% of its services and running an emergency timetable during the dispute.
Marc Reddy, managing director of the firm, said: "We simply cannot understand the rationale for further strike action.
"Typically, when both parties agree to resolve a dispute through collective arbitration, as in this case, industrial action ceases, on the basis that the arbitrator will resolve the dispute with a decision.
"The decision to strike this week and next is completely unnecessary and will have no bearing on an arbitrator's decision."
Unite regional officer Bob Lanning said: "We are keen to get the arbitration process rolling, but the company has yet to agree the terms of reference."
He added drivers "won't even consider suspending the strikes until we have a copper-bottomed agreement for a date for the Acas talks".
Bus drivers in Dorset who are locked in a dispute over pay are to extend their strike, a union has confirmed.