American Airlines president takes same job with United

American Airlines President Scott Kirby has taken the same job at a rival airline.

His switch to president of United Airlines is effective immediately.

In a letter to employees, United CEO Oscar Munoz said that "will allow me to sharpen my own focus as CEO on the core mission of driving United's overall strategy, business innovation and financial performance."

Kirby also will receive nearly $3.9 million in severance under a "transition and separation agreement" from American.

American promoted Robert Isom, who had been the No. 3 executive at the airline, into Kirby's job.

Read more from CNNMoney.