U.S. citizens are divided over the August 1945 atomic bombings of Hiroshima and Nagasaki in the closing days of World War II, an opinion poll conducted by the Pew Research Center in the United States shows.

Respondents who said the bombings were "justified" accounted for 35%, while 31% said they were "not justified." Those who were "not sure" made up 33%.

The survey, released Monday, was conducted on more than 5,000 U.S. adults between June 2 and 8, ahead of the 80th anniversary of the U.S. atomic bombings on the city of Hiroshima on Aug. 6, 1945, and the city of Nagasaki three days later.

The share of respondents who answered that the attacks were justified fell sharply from 56% in the previous survey, in 2015, and that of those who said they were not justified also dropped, from 34%. The decreases in both answers were apparently due to the "not sure" option not being available in the 2015 survey.

By gender, 51% of the male respondents thought the bombings were justified while the proportion stood at only 20% for female respondents.

Older respondents tended to defend the use of the atomic bombs, with the proportion of those age 65 or over who are supportive of the bombings coming to 48%. By contrast, only 27% of those under 30 gave that answer.

Meanwhile, 69% of all respondents think that the development of nuclear weapons has made the world "less safe," far exceeding the 10% who answered the opposite.

A large majority, at 83%, of all respondents said they "know a lot” or “a little" about the atomic bombings of Hiroshima and Nagasaki.