By Brendan O'Brien

CHICAGO (Reuters) - The Chicago Teachers Union summoned hundreds of its leaders for a Tuesday evening update on a nine-day-old strike but said negotiators had not yet reached a tentative deal to end the walkout in the third-largest school district in the United States.

The union said the 825 members of its House of Delegates would meet behind closed doors at 6 p.m. (2300 GMT) for the first time since walking off the job on Oct. 17, canceling classes for some 300,000 students.

Chicago's 25,000 public school teachers want a pay increase along with smaller class sizes and more support staff. They have also put a litany of issues on the table, including the length of the school year and preparation time for teachers.

Mayor Lori Lightfoot said she and Chicago Public Schools Chief Executive Officer Janice Jackson met with union leadership for more than three hours on Tuesday, but both sides did not come to a resolution.

"We made some movement to try to get a deal done and I was deeply disappointed that with that movement ... they would not take the deal," she said during a news conference.

The strike is the latest in a wave of work stoppages across the United States by educators who have called for more resources and emphasized the need to help underfunded schools, framing their demands as a call for social justice.

It is the second-longest U.S. teachers' strike in recent memory. A teachers strike in Union City, California, in June lasted three weeks.

District officials said on Tuesday that they had proposed to spend $25 million to address overcrowding and a further $70 million to hire support staff such as nurses and social workers.

"Chicago is one of the wealthiest cities in the world. CPS has the resources to land a deal that puts our district on the path to real equity for students and the educators who serve them," a union statement said. "The ball is in Mayor Lightfoot’s court."

The union has been without a contract since July 1.

Teachers have picketed in front of many of the district's 500 schools and have held several rallies in downtown Chicago.

The strike is the first major test for Lightfoot, a political newcomer elected in April who campaigned on many of the issues raised by the union. She 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 $7.7 billion school budget.

(Reporting by Brendan O'Brien in Chicago; Additional reporting by Rich McKay in Atlanta; writing by Scott Malone; Editing by Peter Cooney and Grant McCool)