The White House has revealed details about a series of mystery UFOs shot down over North America amid mounting questions, and rising tensions with China.

In a news briefing early Tuesday (Australian time), National Security Council spokesman John Kirby also defended the decision to shoot down the unidentified devices, despite having little information about them.

Mr Kirby revealed the three latest UFOs – shot down over Canada, Alaska and Michigan – were assessed before the take-down decisions were made.

Mr Kirby said the objects appeared to have no communications capability, were not transmitting information, and “there was no reason to suspect” they were being used for surveillance.

However he said surveillance could also not be ruled out.

Mr Kirby said the three mystery devices detected over the weekend were not manned and could not be maneuvered.

He said it appeared as though the objects, which were flying at a lower altitude than the first Chinese spy balloon shot down more than a week ago off South Carolina, posed no direct military threat.

Mr Kirby said while China had admitted to owning the original giant balloon which was spotted floating across the US, the owners of the three latest UFOs were not known.

He said the devices appeared to “move by prevailing winds” as they had no self-propulsion and had been tracking west to east over North American airspace.

Mr Kirby admitted the US still did not know exactly what the three latest devices looked like, their purpose or who owned them.

When asked why a series of mystery flying objects had suddenly been uncovered, Mr Kirby said one reason could be improved radar detection.

Mr Kirby said new parameters on radars were allowing agencies to look more closely at the sky to spot smaller objects, flying at slower speeds and lower altitudes.

Mr Kirby said the US had “acted with an abundance of caution” in shooting down the UFOs with military fighter jets.

Although the objects did not pose a military threat, they could have impacted civilian air space, he said.

He said more would be known about the three unclaimed apparatus when the recovery operation was complete.

The fourth device was shot down on Sunday (local time) when an octagonal object with strings hanging off it was downed over Lake Huron in Michigan near the border with Canada.

Mr Kirby said the recovery operation was hampered by the remote location and conditions such as ice, winds and wilderness.

When asked if it was strange the three objects had not been claimed Mr Kirby said: “We’re sort of in uncharted territory here, no pun intended, we don’t know.”

US accused of flying balloons

Earlier, Washington flatly rejected China’s counter-accusation that the US has been flying high-altitude balloons over China airspace.

Beijing claimed the US illegally flew balloons over its airspace more than ten times over the past year.

“Since last year, the US’s high-altitude balloons have undergone more than 10 illegal flights into Chinese airspace without the approval of the relevant Chinese departments,” Chinese foreign ministry spokesperson Wang Wenbin said at a regular briefing in Beijing in response to a question.

Mr Wang did not specifically describe the balloons as military, or for espionage purposes and did not provide further details.

The White House promptly denied China’s accusation, which National Security Council spokeswoman Adrienne Watson dismissed as an effort at damage control by Beijing.

“Any claim that the US government operates surveillance balloons over the PRC (People’s Republic of China) is false,” she said in a statement.

“It is China that has a high altitude surveillance balloon program for intelligence collection, connected to the People’s Liberation Army, that it has used to violate the sovereignty of the United States and over 40 countries across five continents,” Watson added.

China has failed to offer “any credible explanations” for the intrusions, she added.

The US shot down what it called a Chinese surveillance balloon off the coast of South Carolina on February 4 after it had drifted across the continental US for days.

China said the balloon was a civilian research craft that had mistakenly blown off course and accused the United States of overreacting.

In recent days, the US military has shot down three other flying objects over North America, most recently on Sunday, when an octagonal object was downed over Lake Huron, the Pentagon said.

Mr Wang said he had no information on the latest three objects shot down by the United States.