"You can rest assured that I will always provide you informed, candid and impartial military advice," he said after being sworn in in Trump's presence.

In concluding remarks, Trump responded: "You're my friend, you're my advisor and you deserve this position. I never had a doubt."

A former ice hockey player, the 61-year-old Milley served in Iraq and Afghanistan and held major commands before becoming chief of staff of the army in 2015.

He is a graduate of Princeton University, earned a masters in international studies from Columbia University and attended MIT's National Security Studies Program.

He succeeds Marine General Joe Dunford, who was named chairman of the Joint Chiefs by former president Barack Obama in 2015 but kept by Trump for a second two-year term.

Milley benefits from a change in the law that extended the chairman's term to four years.

General James McConville replaced him as head of the army in August.