Washington: The US defense secretary on Monday defended the decision to sack his navy secretary, saying he went behind his back to make a deal with the White House over a convicted Navy SEAL's future.

Mark Esper told reporters that Richard Spencer, the Navy's top civilian, admitted to him that he had gone around Esper and Joint Chiefs Chairman Mark Milley.

"We were completely caught off guard by this information, and realised that it undermined everything we have been discussing with the president."

Spencer was fired on Sunday amid a White House-Pentagon dispute over whether the Navy SEAL, accused of war crimes in a high-profile case but found guilty of a lesser offense, should be expelled from the elite commando force.