Carl Icahn increased his stake in Navistar International Corp (NAV.N) to 13.19 percent as of July 11, up from his previous stake of 11.87 percent.
The move comes a day after MHR Fund Management LLC -- largest shareholder in the U.S. truck and engine maker -- raised its stake to 14.95 percent.
Icahn, a billionaire investor known for shaking up companies and advocating sales, had pushed for a merger between Navistar and rival Oshkosh Truck Corp (OSK.N) early this year.
Navistar has been struggling for the past year to contain costs of developing a new type of diesel engine for heavy trucks, and has seen its shares lose about half their value in the meantime.
The company, facing pressure from investors to sell itself or change its engine strategy, said last week it was developing a new engine which is expected to be ready early next year.
(Reporting by Bijoy Koyitty in Bangalore)