"We are communicating with the United States but we decline to comment on diplomatic exchanges," chief government spokesman Hirokazu Matsuno told reporters Thursday.

"Having said that, we are analysing objects observed above Japan in June 2020 and September 2021, including any links to the case in the United States."

A mysterious balloon-like object was spotted over northern Japan in June 2020, with locals posting pictures on social media.

Authorities said then that they were baffled by the object, which in close-up images by residents and media appeared to be composed of a balloon attached to crossed sticks with propellers.

Japan's meteorological agency said at the time that the balloon looked like a weather-monitoring device but did not belong to them.

The government batted away suggestions that it might belong to a foreign government, but the recent furore over an alleged Chinese surveillance balloon shot down after passing over the US has prompted a re-evaluation.

Matsuno also revealed that "the government has spotted similar balloons whose affiliation are unknown", including in January 2022 over the sea west of Kyushu, in southwestern Japan.

"We continue to make the utmost efforts to collect and analyse (information) in cooperation with our ally," he said.

Japan's defence ministry said earlier this week that it conducted "24-hour, 365-day surveillance of the airspace over Japan".

But it declined to confirm whether balloons like the one shot down by US forces had been observed over Japan.

"However, when we confirm a case of airspace invasion, we make announcements appropriately. We have never confirmed or made an announcement on any airspace violation by balloons," the ministry said in response to a question by AFP.