"North Korea fired an unidentified projectile into the Sea of Japan this morning from (the eastern port of) Sinpo," the ministry said in a statement.
If confirmed to be a missile, it would be the latest such launch by Pyongyang and will fuel international concerns about the isolated regime's military ambitions.
It came after President Donald Trump threatened the United States was prepared to go it alone in bringing the North to heel if China did not step in, and ahead of a meeting between Trump and President Xi Jinping.
North Korea's foreign ministry on Monday assailed Washington for its tough talk and for an ongoing joint military exercise with South Korea and Japan which Pyongyang sees as a dress rehearsal for invasion.
The "reckless actions" are driving the tense situation on the Korean peninsula "to the brink of a war", a ministry spokesman was quoted as saying by the official KCNA news agency.
The hardened US stance followed two North Korean nuclear tests last year and recent missile launches that Pyongyang described as practice for an attack on US bases in Japan.
Pyongyang is barred under UN resolutions from carrying out ballistic missile launches or nuclear tests.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)