United Airlines Names Acting Chief Exec

United Airlines has appointed an interim leader four days after chief executive Oscar Munoz suffered a heart attack, raising uncertainty about whether he would return.

The airline said its general counsel Brett Hart, 46, would take on the role of acting chief executive, effective immediately. It said it is too soon to know the course of treatment Munoz will take or the timing of his recovery.

The news marks the second CEO shake-up at United since early September. In a news release, Hart vowed to continue an agenda set by Munoz that has been aimed at rebuilding the company's morale after years of strained labour relations.

United said in a statement that its board "remains actively engaged in preparing for all potential outcomes regarding the company's leadership structure." A spokeswoman declined to elaborate.

Hart will make his first presentation to investors as acting chief executive on Thursday, when the company reports third-quarter results.

Munoz, 56, became CEO in September after predecessor Jeff Smisek resigned in relation to internal and federal investigations into United's relationship with the Port Authority of New York and New Jersey.

Munoz spent his first month on the job traversing the United States to meet workers and customers for input on how to improve the airline. Unions have welcomed Munoz, hoping he would push the company to conclude contracts with flight attendants and maintenance workers that have been stalled for years.