"All DoD personnel, service members, and civilian employees alike, swear an oath to protect and defend the Constitution of the United States," Shanahan said in the memo, which was sent to all Pentagon employees and military officials abroad.

"Our mission, to protect and defend the nation, is apolitical."

Shanahan confirmed Sunday that the White House had requested that the USS John S. McCain be kept out of sight during a visit to Japan by President Donald Trump, a political rival of the late senator.

The president's staff had contacted the Japan-based 7th Fleet of the US Navy and given "the directive that the USS John S. McCain should be hidden from view."

When asked about the memo, Shanahan told reporters, "What I wanted to do is, after the McCain situation, remind everyone that we're not going to politicize the military."

"Think of all the travel that's going to come up. Think of the season that we're entering into," he said, referring to the impending 2020 presidential primaries.

"So nothing is wrong with having a reminder."

Trump had a combative political relationship with McCain, who died in August of brain cancer and was taken prisoner and tortured during the Vietnam War.

McCain, a respected figure in US politics for decades who himself ran for president twice, withdrew his support for Trump in the 2016 presidential election and blocked Trump's healthcare efforts in Congress in 2017.

Even after McCain's death, the president has continued to make his dislike clear, saying he was "never a fan of John McCain and never will be."

When the allegations of the White House ordering the warship hidden came to light, Trump insisted he would not have ordered hiding the vessel from his view, adding he "couldn't care less" whether a Navy destroyer bore McCain's name.