The Pentagon said Thursday it was tracking a Chinese spy balloon flying high over the United States, reviving tensions between the two countries just days ahead of a rare visit to Beijing by the top US diplomat.

At President Joe Biden's request, Defense Secretary Lloyd Austin and top military officials considered shooting the balloon down but decided doing so would endanger too many people on the ground, a senior defense official told reporters Thursday.

"Clearly, the intent of this balloon is for surveillance," the official, who spoke on condition of anonymity, said.

The official added that the balloon had flown over the northwest United States, where there are sensitive airbases and strategic nuclear missiles in underground silos, but that the Pentagon did not believe it constituted a particularly dangerous intelligence threat.

"We assess that this balloon has limited additive value from an intelligence collection perspective," the official said.

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The discovery of the aircraft comes just days before an expected visit to China by US Secretary of State Antony Blinken, with managing heightened tensions between the two powers at the top of the agenda.

Blinken's visit to Beijing, which follows a meeting last November between Biden and Chinese President Xi Jinping on the sidelines of the G20 summit, will be the first trip to the Asian country by the United States' top diplomat since 2018.

In addition to ongoing disputes over trade and intellectual property, relations between the two countries have frayed particularly over democratically-governed Taiwan, which China has pledged to reunite with the mainland, by force if necessary.

The United States has been selling arms to Taiwan to defend itself, and Biden has said Washington would help protect Taiwan if China attacked.

The defense official said that the balloon entered US airspace "a couple days ago," but that American intelligence had been tracking it well before that.

Austin, who was in the Philippines, held discussions Wednesday with top Pentagon officials after Biden asked about options for dealing with the balloon.

Fighter jets were flown to examine it while it was above Montana as discussions took place.

- 'Seriousness' of issue -

But the Pentagon decision was "not to take kinetic action due to the risk to safety and security of people on the ground from the possible debris field," the official said.

Pentagon spokesman Pat Ryder confirmed the balloon was still being tracked over US airspace.

"The balloon is currently travelling at an altitude well above commercial air traffic. It does not present a military or physical threat to people on the ground," Ryder said in a statement.

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China has sent surveillance balloons over the United States in the past.

However, this one has lingered in US airspace much longer, the senior defense official said.

"We are taking steps nevertheless to protect against foreign intelligence collection of sensitive information," the official said.

Austin was in the Philippines this week to strengthen US defense cooperation, including gaining wider access for Pentagon forces at Philippine military bases, in a move that highlights the US view of China as a threat to East Asia.

The defense official said the "the seriousness of the issue" with the balloon had been raised with Beijing officials.

"We have made clear we will do whatever is necessary to protect our people in our own land."

Tensions over Taiwan reached a furor last year when Nancy Pelosi, then-speaker of the US House of Representatives, chose to visit the island.

After Republicans gained control of the chamber in January, questions have been raised over whether her successor will make a similar trip.

"China’s brazen disregard for U.S. sovereignty is a destabilizing action that must be addressed, and President Biden cannot be silent," current Speaker Kevin McCarthy tweeted Thursday evening.