FILE PHOTO: Teachers, students and members of the community march toward the Denver Central Library, where contract negotiations between school district and teachers union officials continue, as Denver public school teachers strike for a second day in Denver, Colorado, U.S., February 12, 2019. REUTERS/Michael Ciaglo

(Reuters) - Striking Denver teachers reached a deal with school district officials after a marathon bargaining session early on Thursday to end their walkout, which was in its fourth day, the union said.

The agreement must be ratified by a majority of union members to become effective, and it was not immediately clear when teachers would return to work.