Japan said Thursday it was coordinating with Washington as it analysed unidentified aerial objects spotted over the country in past years after US forces shot down an alleged Chinese surveillance balloon.

"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.

Biden says not seeking conflict with China

President Joe Biden said Wednesday the United States is not looking for conflict with China despite heightened tensions over last week's downing of a Chinese balloon that US officials say was part of a spy fleet spanning five continents.

"We're going to compete fully with China, but... we're not looking for conflict -- and that's been the case so far," he said in a televised interview with PBS.

Pointing to global ramifications of the balloon incident that has animated the United States, Secretary of State Antony Blinken said earlier that the United States was giving data to allies as it assesses recovered debris.

"We already shared information with dozens of countries around the world, both from Washington and through our embassies," Blinken said.

"We're doing so because the United States was not the only target of this broader program, which has violated the sovereignty of countries across five continents," he told a joint news conference with NATO chief Jens Stoltenberg.

White House Press Secretary Karine Jean-Pierre, speaking separately to reporters on Air Force One, described the balloons as part of a fleet and said they had been spotted around the world for several years.

A huge white balloon carrying sophisticated equipment traversed the continental United States last week before Biden ordered the military to shoot it down just off the east coast in the Atlantic.

China insists that the balloon was merely conducting weather research, but the Pentagon described it as a high-tech spying operation. The balloon floated at an altitude far higher than most airplanes and crossed directly over at least one sensitive US military site.

- 'Pattern' by China -

The NATO secretary-general, whose visit to Washington followed a trip to Japan and South Korea, said the balloon showed the need for countries across the alliance to protect themselves.

"The Chinese balloon over the United States confirms a pattern of Chinese behavior where we see that China over the last years has invested heavily in new military capabilities," Stoltenberg said.

"We've also seen increased Chinese intelligence activities in Europe. They use satellites, they use cyber and, as we've seen over the United States, also balloons. So we just have to be vigilant," he said.

Stoltenberg also issued a new warning that China was drawing lessons from the war in Ukraine, which NATO countries have been supporting as it fights back Russian invaders.

"What happens in Europe today could happen in Asia tomorrow," Stoltenberg said, pointing to China's pressure on Taiwan, the self-governing democracy claimed by Beijing.

The Washington Post, quoting US officials, reported that the Chinese surveillance program has been run partly out of the southern island of Hainan.

Updating old technology, the balloons have been used to monitor the military assets of nearby rivals such as Japan, India and Taiwan, the newspaper said.

Biden, in his State of the Union address on Tuesday, said that his orders to shoot down the balloon showed that the United States would not hesitate to act when needed.

But he also stressed that the United States did not want conflict with China. The balloon episode led Blinken to postpone a trip to China in which he had hoped to work at easing tensions.

China, which voiced regret about the air intrusion but later denounced the US decision to down it, responded to Biden that it would also "firmly defend" its interests.

US officials said they took measures to prevent the balloon's instruments from collecting confidential information during its flight.

They say that the decision to wait until the balloon reached water was the only way to ensure the safety of people on the ground.

Biden's Republican rivals say he should have ordered the balloon to be destroyed as soon as it was identified.