According to the Pentagon, a Chinese spy balloon has penetrated US airspace in order to scout out highly sensitive nuclear weapons bases.
The balloon is being closely monitored, a senior Defense Department official said on Thursday.
A possible launch was discarded because it might have endangered too many people.
The risk of being spied on is relatively low.
The "relatively large" balloon entered American airspace a few days ago and is flying at high altitude over the northwestern United States, the official, who asked to remain anonymous, said.
His path had previously been followed by the military.
"The aim of the balloon is clearly espionage and its current path takes it through sensitive bases," said the Pentagon official.
These include air force bases and underground missile sites.
"We have no doubt that the balloon comes from China," it said.
"We are taking measures to protect ourselves against the collection of sensitive information." However, according to the Pentagon, the danger is not particularly great.
"We have come to the conclusion that this balloon has limited capabilities from an espionage perspective."
After the discovery of the balloon, President Joe Biden reportedly ordered that a possible launch be examined.
Secretary of Defense Lloyd Austin and top military officials ultimately decided against it because too many people on the ground could be endangered if shot down by the falling parts.
"In our estimation, it is so large that its debris could cause damage." According to the information, fighter jets flew near the balloon over the state of Montana in order to be able to assess it.
Pentagon spokesman Pat Ryder later confirmed that the balloon was still flying over American soil.
He flies "well above the level of air traffic" and "poses no military or physical danger to people on the ground".
According to the Pentagon official, who spoke anonymously, China has already been contacted about the incident and Beijing has been "clarified the seriousness of the incident."
"We have made it clear that we will do whatever it takes to protect our people and our country."