Updated 09/09/12 – 3:45 p.m.

CHICAGO (CBS) – Chicago Public Schools officials and teachers continued contract negotiations Sunday morning, in talks union officials described as “intense,” with a midnight deadline looming for a teachers’ strike.

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CPS and Chicago Teachers Union negotiators met at the union’s headquarters at the Merchandise Mart at 11 a.m. to resume contract talks. If a deal is reached before the midnight deadline for a teachers’ strike, the union has said it would quickly gather delegates to vote on whether to call off their plans for a walkout.

The union has set a Monday deadline for a strike, which would be the city’s first since 1987.

Sunday afternoon, union spokesperson Stephanie Gadlin told WBBM Newsradio “Negotiations are intense, and they are still ongoing. We should know something by the evening.”

Chicago Board of Education President David Vitale called Saturday’s session a “productive day” and said CPS had addressed several concerns of the Chicago Teachers Union, including yearly pay increases for experience and recall rights for laid-off teachers.

“We have high hopes when we get back together tomorrow we can walk on to the end game,” Vitale told reporters outside the Merchandise Mart, where both sides met in a fourth-floor office.

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He added: “I am still optimistic our children will be in school on Monday morning.”

CTU President Karen Lewis agreed progress had been made but said gaps remain in agreements on compensation, job security and other issues. She said teachers representatives received the latest offer from CPS toward the end of Saturday’s session and needed to absorb the proposal before reconvening at 11 a.m. Sunday.

“It’s an improved offer but I don’t know that I would use the term ‘dramatically,'” Lewis said.

The clock is ticking. CTU delegates recently voted to strike at 12:01 a.m. Monday if a contract is not hammered out. That would deny classroom instruction to 350,000 students across the city and place 30,000 teachers on picket lines.

Lewis said union delegates can meet on short notice to call off the strike if a contract is hammered out Sunday.

CPS has planned for a strike and will open some buildings to students to give them activities to do. Lewis says parents should make alternate plans.

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Charter schools are not affected by the potential strike.