(Reuters) -Elon Musk's X illegally fired an employee in retaliation for her internet posts challenging its return-to-office policy, the U.S. labor board alleged on Friday.

In the complaint, a regional director of the U.S. National Labor Relations Board (NLRB) accused X — formerly known as Twitter — of violating the federal law that prohibits punishing employees for communicating and organizing with others about their working conditions.

X did not immediately respond to a Reuters request for comment.

The NLRB said the dispute began on Nov. 10, 2022, after Musk ordered workers back to the office last November and reportedly said "if you can physically make it to an office and you don't show up, resignation accepted".

Employee Yao Yue responded with a post on Twitter telling fellow workers, "Don't resign, let him fire you." A few days later, she was terminated in violation of the National Labor Relations Act, according to the complaint.

Musk completed his $44 billion acquisition of Twitter last October and began his ownership with a round of firings, including top executives, laying off more than half of its workforce as a cost-cutting measure.

The company is facing a series of lawsuits stemming from the layoffs, including claims that it targeted women and workers with disabilities and that it failed to pay promised severance to laid off employees.

The company has denied wrongdoing in the cases in which it has filed responses.

(Reporting by Samrhitha Arunasalam in Bengaluru and Brendan Pierson in New York; Editing by Maju Samuel, Alexia Garamfalvi and Josie Kao)