US entertainment giant Disney was forced to apologise after one of its Japanese language tweets was criticised for being insensitive as the country commemorated the 70th anniversary of the Nagasaki atomic bombing.

The company's Japanese unit, referencing the animated film "Alice in Wonderland", took to social media on Sunday with a message about the movie's well-known "unbirthdays".

The original English tweet wished followers a "very merry unbirthday".

In Disney's original film, characters celebrate an "unbirthday" -- every day of the year except their own birthday -- during a party with Alice.

However, the Japanese tweet's wording translated as "a day of nothing" -- and coincided with commemorations for the more than 74,000 people killed when the US dropped an atomic bomb on Nagasaki.

Disney quickly deleted the tweet on Sunday, following a string of online criticism.

"We deeply apologise for using an inappropriate expression that could annoy people," it tweeted on Sunday.

Company officials could not be reached for comment on Monday.

The attack on Nagasaki came three days after American B-29 bomber Enola Gay dropped a bomb, dubbed "Little Boy", on Hiroshima, the first atomic bombing in history.

The twin bombings were the final chapters of World War II, with Japan surrendering on August 15, 1945.