The Chicago Teachers Union rejected a new proposal from Mayor Lori Lightfoot on Friday that appeared to compromise on some issues but not budge on many key union demands

After the city and union met for nearly eight hours Friday, both sides came out firing with attacks that escalated tensions with the CTU set to strike Thursday. The contentious bargaining session also appeared to douse some of the growing optimism behind closed doors that a deal could be made in the coming days.

In a statement, the mayor said “it appears that CTU is refusing to negotiate in good faith, and instead is determined to strike at all costs.”

Lightfoot said Chicago Public Schools’ 72 pages of proposals moved the city closer to the union’s demands, offering to: drop a proposal to reduce teacher preparation time; maintain a moratorium on charter schools; include supports for homeless students.

The city’s bargaining team, however, was “very disappointed” by the union response: a three-page document rejecting 16 of the city’s 19 proposals and referred back to CTU demands from January, according to a source close to negotiations.

CTU President Jesse Sharkey told reporters an hour later that CPS gave a “take it or leave it offer” that was “a joke” and didn’t go nearly far enough in addressing the union’s central demands.

Stacy Davis Gates, CTU vice president, said the union would have no choice but to strike if the proposal really was the city’s final offer — and questioned whether the mayor was being “petty that this union didn’t endorse her.”

CTU bargaining team member Katie Osgood said some offers were “offensive,” such as a “paltry” $400,000 per year CPS said it would set aside to build a pipeline of nurses, counselors and case managers.

Another $1 million offered over five years to help reduce class size in grades 4-12 was “a joke,” Sharkey said, though he noted the city was wiling to keep discussing class size and staffing.

“We’re talking about an offer that frankly was insulting to us, and when you add that to the fact that they told us this is our final offer, take it or leave it, that draws the line in the sand for us,” Sharkey said.

In a joint statement, Lightfoot and schools chief Janice Jackson said “we have bent over backwards to meet CTU’s concerns.”

“Unfortunately, CTU simply rejected our proposals without a counter-proposal, and instead is continuing to stand on their $2.5 billion starting proposal from January on nearly all issues.”

Negotiations are set to resume Saturday, though nothing is scheduled for Sunday.

The clock is ticking; the CTU’s 25,000 members, along with 10,000 school support staff and Chicago Park District workers represented by SEIU Local 73 are set to strike Thursday.

Lightfoot and Sharkey have both said an agreement must be reached early next week to avert a strike. The city assumes that means by Monday or Tuesday at the latest, a source said.