Washington DC - US President Joe Biden said he made no apologies for shooting down what was a suspected spy balloon from China, slamming the incursion into US airspace in comments on Thursday.

The "violation of our sovereignty is unacceptable," he said and shooting it down sent a "clear message."

"I hope we have we are going to get to the bottom of this but I make no apologies for taking down that balloon," he said, adding that he expected to talk to Chinese leader Xi Jinping to clarify the issue.

The US military shot down what was thought to be a Chinese spy balloon that had drifted over the state of Montana at the start of the month, causing the rift between Beijing and Washington to deepen.

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US Secretary of State Antony Blinken canceled a trip to Beijing in response.

China insists the balloon was a scientific aircraft and that the US had overreacted.

However, three airborne objects that were more recently shot down by the US military were most likely research vessels, Biden said, and not espionage aircraft as feared in the aftermath of the Chinese balloon.

The aircraft shot down over Canada, Alaska, and Lake Huron probably belonged to research companies, according to intelligence analysts' assessments, Biden said.

"We don't yet know exactly what these three objects were," he said. "But nothing right now suggests they were related to China's spy balloon program, or there were surveillance vehicles from any other country."

They were most likely balloons tied to private companies, recreation, or research institutions, studying weather or conducting other scientific research, he said.