The White House said China has a high-altitude balloon programme for intelligence collection, which is connected to its army.
The US National Security Council's John Kirby said he could not "publicly go into many details", but the programme was not detected by Donald Trump's administration.
He added that America and Canada have now been scrutinising airspace more closely.
He said the UFOs were not aliens but, because the US has not been able to definitely say what the objects are, they are being shot down.
After being asked by reporters why the objects are being shot down if they do not pose a military threat, Mr Kirby said there is a "very real" potential risk to civilian air traffic.
He added that President Joe Biden directed the military "out of an abundance of caution" to take them down as it could not be ruled out whether the objects were watching military sites.
Yesterday, a US fighter jet show down an "unidentified object" over Lake Huro on order of the president.
It was the fourth such downing in eight days and the latest military strike in an extraordinary chain of events over US airspace that Pentagon officials believe has no peacetime precedent.
Tonight, Kirby said the object is likely lying in very deep water.
Part of the reason for the repeated shootdowns is a "heightened alert", following a spy balloon from China that emerged over US airspace in late January, Gen. Glen VanHerck, head of NORAD and US Northern Command, said in a briefing with reporters.
Since then, fighter jets last week also shot down objects over Canada and Alaska.
Pentagon officials said they posed no security threats, but so little was known about them that Pentagon officials were ruling nothing out.
Melissa Dalton, assistant defense secretary for homeland defence, said: "We have been more closely scrutinizing our airspace at these altitudes, including enhancing our radar, which may at least partly explain the increase."
China's Foreign Ministry said the unmanned balloon was a civilian meteorological airship that had blown off course. Beijing said the US had "overreacted" by shooting it down.
Mr Kirby added the US is not flying surveillance craft over China, saying: "We are not flying surveillance balloons over China. I'm not aware of any other craft we're flying into Chinese airspace."
