The US Department of Defense says China is carrying out a large-scale surveillance program by flying balloons over the United States and elsewhere.

The US military on Saturday shot down a Chinese balloon off the Atlantic coast, after it had flown across the US mainland.

The department says there have been at least three other balloon incidents during the previous administration of President Donald Trump and one other incident during the current administration.

Department Press Secretary Pat Ryder told reporters on Wednesday that they flew over sites that would be of interest to China.

He added that balloons have been confirmed over East Asia, Southeast Asia, Latin America and Europe, in what he described as a "larger Chinese surveillance balloon program" that has been operated for several years.

US Secretary of State Antony Blinken said at a news conference on Wednesday that the US has shared information about the downed balloon with dozens of countries around the world.

He stressed that the US continues to look to China to act responsibly, and to abide by international law and stop violations of sovereignty.

NATO Secretary General Jens Stoltenberg, who joined Blinken at the news conference, said, "We need to be aware of the constant risk of Chinese intelligence and then step up what we do to protect ourselves."