TOKYO: A letter threatening to hunt Koreans and containing what appeared to be a bullet has been sent to the South Korean embassy in Japan amid worsening ties between the Asian neighbours, Japanese media said on Tuesday.

Relations between the two countries have been overshadowed by Japan's 1910-1945 colonisation of the Korean peninsula, and recently a dispute over Korean forced labourers spilt into trade and then into security when South Korea scrapped an intelligence-sharing pact last month.

"I've got a rifle and I'm hunting Koreans," said the letter that was delivered to the South Korean embassy in Tokyo last week, the Kyodo news agency reported.

It contained what appeared to be bullet, the news agency said, adding that police were investigating.

Police declined to comment.

A member of staff at the embassy confirmed that the letter had been delivered but declined to give any details.

The tension between the two countries has spilt over into travel and culture, with a Japanese airline announcing last week it would halt some flights to South Korea.

A Japanese art exhibition has withdrawn a statue by Korean artists representing Korean women forced to serve in Japan's military brothels during World War Two, sparking debate about censorship.

Publishers of Japan's weekly Shukan Post tabloid apologised on Monday after their September 13 edition, which carried a special report titled "We Don't Need Korea," sparked widespread outrage and accusations of hate speech.

"This report will spread misunderstanding and was lacking in consideration," the magazine's editors said in a statement.

Though many Twitter users denounced the magazine with comments such as "We Don't Need Shukan Post", others defended it, saying Koreans and their supporters were being too sensitive.