ATLANTA - The chief of the world's biggest airline is encouraging his employees to call police if they feel threatened by union representatives he says are showing up at their homes unannounced.

Delta Air Lines' two biggest work groups -- ground workers and flight attendants -- have yet to resolve representation following Delta's 2008 acquisition of Northwest Airlines. Both groups were nonunion at pre-merger Delta, but union at pre-merger Northwest.

CEO Richard Anderson told employees in a recorded message Thursday that some workers claim International Association of Machinists and Aerospace Workers representatives have frightened their spouses or children and, in some cases, blocked their driveways, preventing them from leaving.

"Conduct like this is harassment. It's wrong," Anderson said. "This is not how we treat people at Delta."

IAM spokesman Joseph Tiberi denies those allegations. He said IAM reps have been respectful to workers, and he accused Delta of impeding efforts by the IAM to meet with employees in their workplace.

"If an employee did not want to speak with us, we moved on," Tiberi said of the home visits.

For unions that represented Northwest workers before the merger to represent workers in their work group at the combined airline, an election must be held and the union approved. Approval requires a majority vote of the entire work group, with those who don't vote counted as "no" votes.