The Pentagon made the assessment in a letter to two House members who asked about "expected casualty assessments in a conflict with North Korea", including civilians and USA troops in South Korea, Japan and Guam, according to the daily.
Trump has rattled some USA allies with his vow to "totally destroy" North Korea if it threatens the United States, for deriding Kim as a "Rocket Man on a suicide mission" and for dismissing as pointless any diplomatic engagement with Pyongyang.
Recent drills over South Korea by two U.S. strategic bombers have further raised tensions. "It's given North Korea a reason to say, 'Hey, we need a bomb, because if we don't have a bomb we're going to not be able to protect ourselves and they'll come after us [sic]'".
North Korea's last nuclear and missile tests both occurred in September.
They said South Korea's promises that it will not deploy additional parts of the THAAD system, will not join the U.S. Trump is looking to present a united front with Japan against North Korea through meetings with Japanese Prime Minister Shinzo Abe.
Trump will steer clear of the heavily fortified demilitarized zone (DMZ) zone on the border between the two Koreas - where other US presidents have visited - a move that might have been seen by North Korea as more of a provocation.
US ally South Korea, meanwhile, is rolling out the red carpet for Trump but is also sensitive to the fact that he plans to spend only 24 hours in the country compared to 48 hours in Japan, the kind of comparison that South Koreans often make because of concerns they are a lesser USA priority than Tokyo, its richer neighbor.More news: Nine-year-old boy with cancer wants cards for his last Christmas
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His administration also planned to take a different approach to dealing with the issue after years of what he termed "total weakness", although he did not give any details.
"This is a country that grew out of the total devastation of the Korean War, and over a matter of six, seven decades we've become a thriving democracy. and a thriving market economy", the foreign minister said.
However, with North Korea expected to be on top of the agenda, tensions are rising in South Korea ahead of Trump's visit.
The US president, according to the newspaper, is "unstable". He said of the 300,000 people who could potentially die in the initial days, more than 100,000 could be Americans.
North Korea has said in public statements that it wants an official end to the Korean War.
While repeatedly calling for dialogue to resolve the issue, China has also signed up for increasingly tough United Nations sanctions against North Korea.
Before Trump's presidency, many in South Korea and the United States regarded the deal as a key pillar of their alliance.