As President Donald Trump’s national security adviser, retired Lt. Gen. Mike Flynn promoted a controversial private-sector nuclear power plan in the Middle East that had once involved Russian companies, according to former security-council staffers and others familiar with the effort.

While working at the White House, Mr. Flynn advocated for a group of former senior U.S. military officers with whom he had worked while in the private sector. The project, which the former military officers were helping promote on behalf of several U.S. companies, envisions building and operating dozens of nuclear plants in Saudi Arabia and across the Middle East, the people familiar with it said.

The sprawling construction project was valued at hundreds of billions of dollars and described as a Marshall Plan for the region, according to the people familiar with it. Mr. Flynn, as a private citizen before entering the White House, had advised U.S. companies that aimed to provide security for the project.

White House disclosure forms indicate that Mr. Flynn’s year-and-a-half work on the project ended in December 2016, but Mr. Flynn in fact remained involved in the project once he joined the Trump administration in January, discussing the plan and directing his National Security Council staff to meet with the companies involved, the former staffers said.

A lawyer for Mr. Flynn declined to comment, as did a White House spokeswoman.