Activist investor Carl Icahn blasted back at the board of Navistar International Corp. on Tuesday, demanding four seats on the embattled U.S. truck and engine maker's board and criticizing the company's description of his campaign as "threats."

"My comments are not threats, they are demands that I take very seriously in light of my substantial investment, and as should you, in light of your fiduciary obligations," said Icahn, the company's third-largest shareholder, with a 14.9 percent stake.

The Lisle, Illinois-based company, which on Monday had dismissed an open letter Icahn wrote to its board and shareholders as "threats, attacks and disruptions," did not immediately respond to Icahn's new missive.

Navistar last month named a new CEO, Lewis Campbell, who is charged with returning the company to profitability after a failed effort to develop a new style of diesel engine failed to win U.S. regulatory approval.