Fairfax Connector bus operators could go on strike as soon as Thursday, as labor negotiations have once again stalled, union leaders said.

Amalgamated Transit Union Local 1764, which represents more than 500 Fairfax Connector employees, called the past several weeks “fruitless” as union representatives try to reach a new collective bargaining agreement with Transdev, the private contractor Fairfax County has hired to run the bus operation.

It would be the second strike in about three months for the workers, and Virginia’s largest bus system, which carries an average of 30,000 riders on 91 routes weekdays.

In early December, Connector workers walked off the job after their last contract expired without a new deal being reached. The strike limited Connector service to about 15 routes before workers ended picketing, and returned to work four days later after Fairfax County elected officials intervened in the negotiations.

ATU International President John Costa said in December the assistance of Fairfax County supervisors “helped us make significant advances at the table.

“We do reserve the right to walk off the job again if the good faith bargaining by Transdev disappears,” he said.

The union said Monday that Fairfax County officials had pledged to help with the labor discussions and push Transdev toward an agreement or hold a vote declaring the multinational transit operator in breach of its contract, “neither of which occurred,” ATU Local 1764 said in a statement.

“Transdev has once again pushed our members to the brink,” Costa said. “While we remain committed to negotiating with Transdev to reach a fair and just contract, we are disappointed that Fairfax County hasn’t stepped in as promised to ensure Transdev gives our members — who serve their community — the fair contract they deserve.

“We are once again left with no other option but to strongly consider walking off the job again later this week,” he said.

Transdev said union officials’ notice of their intent to possibly strike came before scheduled negotiation sessions — that had been mutually agreed upon for more than a month — took place Monday, Tuesday and Wednesday.

“We were disappointed to receive notification from the ATU of their intent to exercise their right to strike,” Transdev spokeswoman Mitun Seguin said. “Transdev believes a fair and equitable agreement can be reached through good faith negotiations without the need to resort to a service disruption.”

Fairfax County Board Chair Jeff C. McKay (D-At-Large) said he is glad both sides continue to negotiate this week, and he reiterated supervisors’ pledge to remain involved in the dispute the best they can.

“Fairfax County is committed to continuing to foster conversations and look at all options moving forward,” McKay said in a statement. “Though we’re constrained in what and how we can get involved in this private process, we’ve been successful in helping to keep buses on the road, and I am hopeful that will continue.”