Washington (CNN) White House press secretary Sarah Sanders said Thursday President Donald Trump was using "common courtesy" when he returned a military salute from a North Korean general during Tuesday's summit in Singapore.

"It's a common courtesy when a military official from another government salutes, that you return that," Sanders told CNN's Jim Acosta during Thursday's White House press briefing.

For US military officers, returning a salute from a military officer of a friendly foreign nation is common practice and considered a display of military professionalism. However, there is no rule that a US president is obliged to return a salute, which is considered a sign of mutual respect.

North Korean state media recently aired footage of Trump returning a salute to a North Korean military general during this week's summit in Singapore -- an extraordinary display of respect from a US president to a top officer of a hostile regime.

After Trump and North Korean leader Kim Jong Un walked down a colonnade to shake hands, the pair entered into a room filled with various members of Kim's delegation, the video from North Korean broadcaster KCTV news shows.

Read More