By the CNN Wire Staff

(CNN) - Tens of thousands of teachers and support staff in Chicago are set to go on strike Monday after their union and school officials failed to reach a contract agreement, the union president said.

"Negotiations have been intense but productive, but we have failed to reach an agreement that would prevent a labor strike," Chicago Teachers Union President Karen Lewis told reporters late Sunday night.

Minutes earlier, the president of Chicago's school board said officials offered the city's teachers a contract including pay increases and other measures they'd requested.

"We've been as responsive as we know how," David Vitale told reporters just before 10 p.m. CT (11 p.m. ET) Sunday.

By the CNN Wire Staff

(CNN) - Despite what the head of Chicago's school board deemed a day of "productive talks," the nation's third-largest public school system entered Sunday without a contract with teachers - and with a strike looming.

Teachers and support staff in Chicago set a walkout date Monday, which would mark the first time they have gone on strike in 25 years.

Speaking late Saturday, Chicago Teachers Union chief Karen Lewis said that no action has been taken to alter the teacher's plans not to work next week.

"We've made some progress, but we've still got big issues on the items we've always had big issues on," she told reporters, shortly after walking out of union headquarters with a group singing "solidarity forever."

About 40 minutes earlier, the president of Chicago's board of education told reporters progress was made in extensive talks Saturday, while noting the pressure posed by the fast approaching deadline.