A letter threatening to hunt Koreans and containing what appeared to be a bullet has been sent to the South Korean Embassy in Japan, police said Tuesday, amid worsening ties between the Asian neighbors

Relations between the two countries have been overshadowed by Japan's 1910-1945 colonisation of the Korean peninsula, and recently a dispute over Korean wartime laborers spilled 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.