The United States military on Saturday shot down a suspected Chinese surveillance balloon off the coast of South Carolina, adding to already strained relations between Washington and Beijing.

A spy balloon, which was used extensively during the cold war, has sophisticated cameras and imaging technology to collect information. It operates quite high in the sky, more or less where commercial aeroplanes are flown.

“The balloon, which was being used by the PRC [People’s Republic of China] in an attempt to surveil strategic sites in the continental United States, was brought down above US territorial waters,” Defence Secretary Lloyd Austin said in a statement.

Austin described the operation as a “deliberate and lawful action” that was taken in response to China’s “unacceptable violation of our sovereignty”.

The balloon first entered US airspace in Alaska on January 28, before moving into Canadian airspace three days later, the BBC reported, citing defence officials. It then re-entered American airspace on January 31 and was seen in the state of Montana, where a number of sensitive nuclear missile sites are located.

Austin said President Joe Biden had issued an order on Wednesday that the balloon be shot down “as soon as the mission could be accomplished without undue risk to American lives”.

Biden later told reporters, “They successfully took it down, and I want to compliment our aviators who did it.”

Beijing, which has insisted that the balloon was merely a weather research “airship” that had been blown off course, on Sunday accused the US of “clearly overreacting and seriously violating international practice”.

In a statement, the Chinese foreign ministry said the government will take necessary action to safeguard its interests.

“China expresses strong dissatisfaction and protests against the use of force by the United States to attack the unmanned civilian airship,” the statement added.