Carl Icahn increased his stake in Navistar International Corp. to 11.87 percent on Thursday, taking advantage of a sharp fall in the truck and engine maker's shares after the company reported a quarterly loss.

Icahn, who had pushed for a merger of Navistar and rival Oshkosh Corp., had earlier reported a stake of 9.99 percent in Navistar.

The billionaire investor bought 883,200 Navistar shares on Thursday, when the company's shares fell as much as 28 percent to $20.21.

Navistar's results have been hit over the last two quarters by warranty costs related to engines built in 2010 and 2011. It is also facing delays in receiving regulatory approval for a new engine that does not comply with current U.S. emission rules.

The company warned investors in March that costs for repairing the engines were taking a heavy toll on profit, but said warranty claims had peaked. Claims, however, have shot up since then.

Navistar Chief Executive Daniel Ustian had said he was open to a merger with Oshkosh, in which Icahn also holds an about 10 percent stake, but the idea was rejected by Oshkosh shareholders.