WASHINGTON—A top federal communications regulator said Monday that he had “serious concerns” about Sinclair Broadcast Group Inc.’s $3.9 billion acquisition of Tribune Media Co. and took action to block the deal.

Federal Communications Commission Chairman Ajit Pai said in a written statement that the evidence suggests that proposed station divestitures by Sinclair to meet FCC media-ownership regulations would leave the company in practical control of those stations “in violation of the law.”

Mr....