Chicago Mayor Lori Lightfoot, a Democrat, and city public school officials walked away from the negotiating table once again Friday amid a standoff over plans to reopen the nation's third-largest district to in-person learning following coronavirus shutdowns.

The Chicago Teachers Union turned down the city’s "last, best and final" offer Friday afternoon, seeking additional concessions from the city, according to union president Jesse Sharkey.

"To say we’re deeply disappointed that the mayor has chosen to end negotiations and instead move to lock out educators and shut down schools rather than work out our differences is an understatement," Sharkey wrote in a statement on the union’s website after rejecting Lightfoot’s offer.

Lightfoot and Chicago Public Schools CEO Janice Jackson in a statement Friday afternoon said that they had not received the union's formal rejection in writing.

"The ball is in their court," they said.

Lightfoot said separately that her plan follows scientific and medical guidelines under the U.S. Centers for Disease Control and Prevention and the city’s Department of Public Health.

"We must respect the tens of thousands of CPS parents — most of them Black and Latinx — who feel their children need in-person instruction," Lightfoot tweeted Friday. "Our plan, endorsed by leading public health experts, gets kids AND staff back safely."

CHICAGO MAYOR TELLS TEACHERS UNION SCHOOLS MUST OPEN NOW, ‘ENOUGH IS ENOUGH’ 

She said reopening schools was "a moral obligation."

Chicago's Mayor Lori Lightfoot speaks during a science initiative event at the University of Chicago in July 2020. (Reuters) (Reuters)

Chicago Public Schools were supposed to partially reopen for children between kindergarten and eighth grade Monday – but union leaders have urged teachers to continue working remotely until a deal is reached.

Jackson noted Friday that the district spent more than $100 million on health precautions and personal protection equipment, yet the union still refuses to return to work.

"Families want to come back to school," she tweeted. "CDC, CDPH, WHO, Academy of Pediatricians, Dr. Fauci, and other leading public health experts have already said we can reopen schools with mitigations in place."

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Fox News’ Greg Norman contributed to this report.