Navistar International Corp. is disappointed by large shareholder Carl Icahn’s campaign of “threats, attacks and disruption” and is committed to its plan to cut costs and review possible asset sales, the U.S. truck and engine maker said on Monday (Sept. 10).

“Navistar maintains an ongoing dialogue with its shareholders, and appreciates their input and views,” the company said in a statement. “As such, after a year of dialogue, we are extremely disappointed that Mr. Icahn has chosen to pursue his unproductive tactics of threats, attacks, and disruption.”

Reuters reported that billionaire investor Icahn slammed Navistar on Sunday for naming a new CEO without consulting large shareholders and urged the company to offer four board seats to shareholders immediately. The Navistar board’s recent decision to appoint Lewis Campbell as chairman and interim CEO was “worse than ill-advised,” Icahn said in an open letter to the directors.

The embattled U.S. truck and engine maker last month replaced CEO Daniel Ustian with Campbell, a former head of Textron Inc. after its bet on a new generation of diesel engines failed to live up to its promise.

“It is … outrageous that you have not reached out to obtain our opinion on issues such as choosing a new management team to lead this company,” said Icahn, who holds a roughly 15% stake in the U.S. truck and engine maker. “This is a board at war with its own shareholders. I urge you to reconsider the path the board has chosen, which harms our company and puts you at serious risk of personal liability.”

The investor said he would prefer to resolve the matter amicably rather than through protracted litigation and a proxy fight, but at the same time is seeking access to corporate documents and board proceedings at Navistar in order to protect his investment.