It's hardly the stuff of little green men, but a mysterious balloon-like object seen floating across the skies of northern Japan has captured national attention, even prompting questions to the government.

The unidentified flying object was first spotted on Wednesday morning when residents in northern Sendai city took to social media to post pictures and debate what they were seeing. "This white thing isn't moving at all. What is it? Can anyone tell me?" wrote one user, with others chiming in using the hashtag "unidentified flying object" in Japanese.

Authorities said they were baffled by the object, which close-up images taken by local residents and media suggest is composed of a balloon-like object attached to crossed sticks with propellers. "The object looks like a balloon for monitoring weather, but it's not ours," an official at the Sendai bureau of the Japan Meteorological Agency told AFP on Thursday.

Local police reportedly flew a helicopter to confirm the presence of the object, but also drew a blank on its identity -- even after consulting other authorities and organisations. Kyushu University's aeronautics department publicly denied the device belonged to them, after public speculation.

Shinichiro Higashino, an associate professor at the department told Fuji TV that a close-up image suggested the object was equipped with solar panels. "It's possible it could conduct scientific observations or monitor something," he said.

The mystery received coverage nationwide, and even prompted a question to the top government spokesman at his daily briefing. Yoshihide Suga said the government was aware of the mystery object, but batted away suggestions it might be the work of foreign governments or be capable of causing damage.

The mystery may remain unresolved -- the object has now reportedly floated out to the Pacific, where authorities have lost track of it.