Facebook Inc. Chief Executive Mark Zuckerberg is taking on Facebook employees who have crossed out the phrase "black lives matter" on the company’s walls and replaced it with "all lives matter."

In a private Facebook post to employees this week, Mr. Zuckerberg called the actions “malicious.” In the post, previously reported by Gizmodo, Mr. Zuckerberg said he told employees at a question-and-answer session last week that such behavior was “unacceptable,” but that it had happened again.

"I was already very disappointed by this disrespectful behavior before, but after my communication I now consider this malicious as well," Mr. Zuckerberg wrote.

He described the incident as “a deeply hurtful and tiresome experience for the black community and really the entire Facebook community.”

Facebook’s headquarters has chalkboard walls in some places and employees and visitors are encouraged to write on them.