Residents of the Japanese coastal town of Hamamatsu have been perplexed by a large iron ball that has washed up on a local beach.

While authorities ruled out the ball containing an explosive device after performing an X-ray exam and finding out it's hollow from within, they remain weary of the object.

At the time it washed up on shore, it was suspected to be a mine or spying device from China or North Korea and the beach was initially cordoned off.

The sphere, measuring about 1.5 metres in diameter, is being inspected by the Japanese army.

At the beginning of the month, the United States downed a suspected Chinese spy balloon off the coast of the Carolinas after it crossed sensitive military sites across North America.

The high-altitude balloon was first spotted over Billings, Montana, and flew over the Aleutian Islands, through Canada, and into Montana.

The Chinese government acknowledged the craft "is from China."

"It is a civilian airship used for research, mainly meteorological, purposes," the Chinese Foreign Ministry said in a statement on its website quoted by NBC News.

Video: The Telegraph