Donald Trump has targeted the United States' relationship with Japan again, this time slamming President Barack Obama on Saturday for not raising the December 1941 attack on Pearl Harbor during the leader's historic visit to Hiroshima the previous day.

"Does President Obama ever discuss the sneak attack on Pearl Harbor while he's in Japan? Thousands of American lives lost. #MDW," Trump, the presumptive Republican presidential nominee, tweeted as the Memorial Day weekend opened in the U.S.

Obama, who became the first sitting U.S. president to visit the A-bombed city, used the trip to discuss the dangers of nuclear weapons in a speech at Hiroshima Peace Memorial Park.