CHICAGO — Thousands of Chicago teachers walked off the job this morning for the first time in 25 years, after union leaders announced they were far from resolving a contract dispute with school district officials.

The walkout in the nation’s third-largest school district posed a tricky test for Mayor Rahm Emanuel, who said he would work to end the strike quickly.

“This is not a strike I wanted,” Emanuel said Sunday night, not long after the union announced the action. “It was a strike of choice ... it’s unnecessary, it’s avoidable and it’s wrong.”

Some 26,000 teachers and support staff were expected to join the picket.

Contract negotiations between Chicago Public School officials and union leaders that stretched through the weekend were resuming today.

Officials said some 140 schools would be open between 8:30 a.m. and 12:30 p.m. so the children who rely on free meals provided by the school district can eat breakfast and lunch, school district officials said.

Union leaders and district officials were not far apart in their negotiations on compensation, Chicago Teachers Union President Karen Lewis said. But other issues — including potential changes to health benefits and a new teacher evaluation system based partly on students’ standardized test scores — remained unresolved, she said.