Canada confirmed Friday that it is working with the United States to track down a high-flying spy balloon and is monitoring a "possible second incident".

"A high-altitude spy balloon has been detected and we are actively tracking its movements," Canada's Department of National Defense said in a statement.

"Canadians are safe and Canada is taking measures to ensure the security of its airspace, including tracking a possible second incident," the ministry said.

The statement added that Canadian intelligence agencies are working with their US partners and continue to take all necessary measures to protect Canada from foreign espionage threats.

While the Canadian statement declined to mention China, the US Department of Defense said earlier that it was tracking a Chinese spy balloon flying at high altitude over the United States, while military officials discussed shooting it down over Montana.

Pentagon spokesman Gen. Patrick Ryder told reporters that the United States had detected and was tracking a surveillance balloon currently flying over the United States.

The US Department of Defense does not believe that the balloon poses a serious intelligence threat, despite its tracking (Reuters)

A US official said that the balloon flew over the northwest of the United States, where sensitive air bases and strategic nuclear missiles are located in underground facilities.

In a later development, the Wall Street Journal quoted a US official as saying that the State Department had summoned the Chinese charge d'affaires in Washington over the spy balloon.

CNN also quoted a US military official as saying that his country had equipped F-22 fighters in Montana, in the event that a decision was made to deal with the Chinese spy balloon.

For his part, a senior defense official told reporters that at the request of US President Joe Biden, Defense Secretary Lloyd Austin and senior military officials discussed shooting down the balloon, but ultimately decided that it could endanger many people on the ground.

But the Pentagon does not believe the airship poses a serious intelligence threat. "We assess that this airship has limited value-add from an intelligence-gathering perspective," the official said.

There was no immediate comment from Beijing on what the US and Canadian sources said about the spy balloon.