Investor Kirk Kerkorian said he bought 100 million Ford Motor Co. shares, or 4.7 percent, and intends to buy more in a vote of support for the chief executive officer, Alan Mulally. Ford rose 10% in New York trading.

The 90-year-old billionaire paid about $691 million so far and plans to spend an additional $170 million. Mr. Kerkorian’s Tracinda Corp. said the second-largest American automaker is “starting to achieve highly meaningful traction in its turnaround efforts.” Mr. Mulally said Ford welcomes the investment.

The offer of $8.50 a share for another 20 million shares reflects confidence in Ford’s progress after it narrowed losses by 50% in the fourth quarter of last year, Tracinda said. The initial investment was made April 2, three weeks before Dearborn, Mich.-based Ford reported a $100 million first- quarter profit that surprised analysts.

“This shows a lot of faith in the revival of the company,” the senior vice president of equity research at National City Corp., in Cleveland, Dan Poole, said in an interview. Because the Ford family controls 40% of the voting shares, “I would have to believe this is a passive stake.”

Three years ago, Tracinda described its first purchase of General Motors Corp. shares as “strictly passive” and voiced support for management. Mr. Kerkorian later built his stake as high as 9.9% as he stepped up pressure on CEO Rick Wagoner to take bold action to transform the world’s largest automaker. Tracinda didn’t characterize its intentions with Ford.