CHICAGO (Reuters) - Union leaders for Chicago’s striking teachers on Wednesday approved a tentative labor deal negotiated with the school district but refused to return to work unless the mayor agreed to make up for instructional days and pay lost during the 10-day walkout.

FILE PHOTO: Teachers protest during a rally and march on the first day of a teacher strike in Chicago, Illinois, U.S. October 17, 2019. REUTERS/John Gress

The Chicago Public Schools district, the third-largest in the United States, issued a separate statement saying the strike would continue, with classes for 300,000 students canceled again, for an 11th straight school day on Thursday.

Mayor Lori Lightfoot immediately rejected the final demand of the 25,000-member Chicago Teachers Union (CTU), accusing its leaders of reneging on the agreement reached at the bargaining table earlier.

“We’ve given them a historic deal by any measure,” Lightfoot said in late-night remarks live-streamed on her Twitter page. “The fact that our children aren’t back in school tomorrow is on them.”

She added: “I’m not compensating for days they were out on strike.”

Terms of the proposed settlement were not disclosed. But some union leaders initially voiced enthusiasm for it.

“The CTU may have reached a monumental agreement,” union Vice President Stacy Davis Gates said earlier on Twitter, referring to the tentative deal.

After huddling in private to review the tentative settlement for several hours, union leaders emerged to call on rank-and-file members to rally on Thursday to press their demand for extending the school calendar to offset days missed during the strike.

“We have a tentative agreement, but we do not have a return-to-work agreement. So we will be at City Hall at 10 a.m. to demand the mayor return our days,” the union said on Twitter.

The walkout in Chicago followed a wave of teacher strikes across the country over wages and education funding during the past two years, including a week-long work stoppage in Los Angeles in January. African-Americans and Hispanics account for the majority of Chicago’s public school enrollment.

As was the case in Los Angeles, the labor dispute in Chicago centered on pay as well as teacher demands for contract language to reduce class size and increase staffing levels for support professionals, including nurses and social workers.

Any settlement is ultimately subject to approval by the union’s House of Delegates, a body consisting of 825 elected representatives from each of the city’s schools and support staff classifications, before classes can resume.

The district had said it was looking into whether it could make up more than eight school days lost during a strike, and the Chicago Board of Education would need to vote on adding any attendance days to the school calendar.

Striking teachers, who have been without a contract since July 1, have picketed daily in front of many of the district’s 500 schools and have rallied several times in downtown Chicago.

The union was seeking a contract that runs three years instead of five and includes more paid preparation time for elementary school teachers.

District officials proposed spending $25 million to reduce overcrowding in the district and another $70 million to hire support staff, such as nurses and social workers.

Lightfoot has said the district could not afford the union’s full demands, estimating they would cost an extra $2.4 billion each year for an increase of more than 30% in the current school budget of $7.7 billion.