A threatening letter containing what appeared to be a bullet was sent to the South Korean Embassy in Japan, police said Tuesday, at a time of sharply deteriorating ties between the two countries over wartime history and trade policy.

Police said they are analyzing the object to confirm whether it is a bullet, and to possibly help identify the sender.

The letter was delivered to the embassy in Tokyo on Aug. 27 and addressed to former ambassador Lee Su Hoon, sources close to the matter said. There was no indication as to who had posted it.

The letter said the sender has a rifle and is targeting a South Korean, according to the sources.

It did not mention the historical issues of "comfort women" -- forced to work in Japanese military brothels -- or wartime Korean laborers during Japan's 1910-1945 colonial rule of the Korean Peninsula, long a source of tension between the two countries.

Relations deteriorated rapidly after the South Korean Supreme Court last autumn ordered two Japanese companies to pay damages to South Koreans who it said were forced to work in their factories during Japanese colonial rule.

Japan maintains the issue of compensation stemming from its colonial rule was settled "finally and completely" in a 1965 bilateral agreement under which Japan provided South Korea with $300 million in grants and $200 million in loans.

Last month, Japan tightened its export controls of materials needed by South Korean manufacturers of semiconductors and display panels, as well as taking South Korea off its white list of preferred trading partners, in what Seoul says is retaliation for the court decision.

In turn, South Korea ended a military intelligence-sharing pact with Tokyo that helps the U.S. allies counter missile threats from North Korea.