Safeway employees at a Denver store walked out Monday, expanding a strike that started this weekend. The strike started small with workers at three stores in Estes Park, Fountain and Pueblo, and a Denver distribution center.

Currently, unfair labor practice strikes are limited to Safeway/Albertsons locations in just four cities: stores in Estes Park, Fountain and Pueblo, and a distribution center in Denver.

Labor negotiations between Safeway workers and the company have stalled in Colorado. A strike could happen as soon as Sunday, June 15.

The union representing Safeway store employees in Colorado has given the required 72-hour notice that the strike is set to begin one minute before midnight on Sunday.

Safeway workers could strike as early as Sunday after the union gave 72-hour notice following stalled negotiations over staffing and unfair labor practices.