The United Teachers Los Angeles declared an impasse Wednesday in its talks with the Los Angeles Unified School District.
The action opens the way for a mediator to be brought in to help bring about a settlement.
Contract talks have been ongoing since July, UTLA said on its website.
"There is still a significant gap between the two sides on compensation," the union stated. UTLA is seeking an 8.5 percent, one-year increase; LAUSD has offered a 5 percent increase.
The union said the district is "refusing to bargain in good faith on student learning conditions, and threatening educator layoffs as a scare tactic."
LAUSD Superintendent Ramon Cortines said in a statement that the district agrees the talks are at an impasse.
"I've been disappointed and frustrated by the lack of progress toward an agreement," he said. "It's my hope that the appointment of a mediator will lead to an expeditious settlement that ultimately supports our students and the District at large."
UTLA represents 31,000 members, including teachers and health and human service professionals.
The differences between the two sides amount to more than $800 million, the district said in its statement. Cortines has maintained that the district is facing a deficit. The union insists the district has money.
Other issues dividing the two sides include class room size and teacher evaluations.