"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.
Trump starts to shake hands with those in the room. Upon encountering North Korean Gen. No Kwang Chol, who was in full uniform, Trump first extends his hand, to which the general responds with a salute. Trump then salutes back, extends his hand again and the two men shake hands.
A US official told CNN that Trump was briefed on protocol, which is to not salute military officers from other countries. The White House, however, is not viewing Trump saluting a North Korean general as a mistake. It is being seen as all part of the broader goal that day, which was to show respect to Kim and his entourage, according to the official.
CNN's Bonney Kapp, Veronica Stracqualursi, and Jennifer Hansler contributed to this report.