Chicago's teachers' union said Sunday night more than 29,000 teachers and support staff would go on strike Monday for the first time in 25 years after contract talks with the school board failed. 

The strike will likely affect about 400,000 public school students.

In a news release Sunday, the union said teachers and the school district had failed to reach an agreement, and the teachers would go on strike. "Pickets are expected to begin Monday at 675 schools and the Board of Education as early as 6:30 a.m.," the statement said.

Chicago Teachers Union President Karen Lewis said in a statement that "negotiations have been intense but productive, however we have failed to reach an agreement that will prevent a labor strike.

"This is a difficult decision and one we hoped we could avoid. Throughout these negotiations have I remained hopeful but determined. We must do things differently in this city if we are to provide our students with the education they so rightfully deserve."

Lewis said she hoped that talks would resume Monday. 

The union has been holding talks with school district officials for months over several contract issues, including better compensation, teacher evaluations and health benefits.  

The school board offered the teachers a 16 percent salary increase over four years and said it was ready to meet most of the demands of the union. The district had changed its proposal 20 times during the talks, but the union did not accept the offers, school board president David Vitale told the Chicago Tribune. 

"This is about as much as we can do. There is only so much money in the system," Vitale told the the Chicago Tribune.

Meanwhile, the Chicago Public Schools district announced on its website that on Monday it would open 144 Children First sites, where parents can bring their children for part of the day.  The school board said it was committed to staying at the negotiating table with the teachers' union "to ensure our students stay in the classroom."