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.

The official said the balloon flew over the US northwest where there are sensitive airbases and strategic missiles in silos underground.

"Clearly, the intent of this balloon is for surveillance, and the current flight path does carry it over a number of sensitive sites," said the official, who spoke on condition of anonymity.

But the Pentagon did not believe it constitutes a particularly dangerous intelligence threat.

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

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

After Biden asked what the options were to deal with it, on Wednesday Austin, who was in the Philippines, held discussions with top Pentagon officials.

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

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.

It was flying at a high enough altitude not to threaten commercial aviation, the official added.

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

However, this one has lingered in US airspace much loner, the official said.

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