The warning was issued this morning following the announcement on Thursday North and South Korea would hold their first summit in more than a decade.
His remarks may be based on satellite imagery provided by the United States.
President Donald Trump will welcome Japanese Prime Minister Shinzo Abe to his Mar-a-Lago club in Florida later this month for a summit meeting, the White House announced Monday. He said the trade deficit between the countries must be corrected, and Japan has not been exempted from Trump's list of countries whose steel and aluminum imports will be subject to tariffs.
There are signs of a thaw on the peninsula, with global attention fixed on two closely related events - an inter-Korean leaders' summit scheduled for April 27, and a first-ever U.S.
Abe said he would ask Trump to bring up the issue of past North Korean abductions of Japanese citizens, during the USA president's expected summit with North Korean leader Kim Jong-un, Reuters reports.
Japan has said North Korea abducted at least 17 Japanese citizens in the 1970s and 1980s to train agents in Japanese language and culture to spy on South Korea.
The prospect of a Trump-Kim summit, as well as talks between North and South Korea, appears to have pushed Tokyo to reassess its position, analysts said.
Abe is also expected to discuss stiff USA tariffs on steel and aluminum imports and urge Trump to exclude Japan, Chief Cabinet Secretary Yoshihide Suga told reporters. Both Abe, who previously visited the resort last February, and Chinese President Xi Jinping have been invited to what the president refers to as "the winter White House".