U.S. Secretary of State Antony Blinken postponed a visit to China that had been expected to start on Friday in response to the Pentagon's discovery of an alleged Chinese spy balloon flying over the continental United States, U.S. officials said Friday.

A senior State Department official told the Washington Post that the presence of the balloon was "unacceptable" and a "clear violation of our sovereignty as well as international law."

The development came just hours before Blinken had been due to depart Washington for Beijing and marked a new blow to already strained U.S.-Chinese relations. There is no new date for Mr. Blinken's trip to Beijing, according to reports.

Discovery of the balloon was announced by Pentagon officials who said one of the places it was spotted was over the state of Montana, which is home to one of America's three nuclear missile silo fields at Malmstrom Air Force Base.

The incident sparked a political furor in the United States, and China on Friday expressed regret that a "civilian" airship had strayed into U.S. airspace after being blown off course.

"The airship is from China. It is a civilian airship used for research, mainly meteorological, purposes," an unidentified spokesperson for China's Foreign Ministry said in a statement on its website. "Affected by the Westerlies and with limited self-steering capability, the airship deviated far from its planned course. The Chinese side regrets the unintended entry of the airship into U.S. airspace due to force majeure."

"Force majeure" refers to a violation caused by forces beyond a party's control.

A senior State Department official told reporters Blinken had nevertheless decided to postpone his trip.

"We have noted the PRC (People's Republic of China) statement of regret, but the presence of this balloon in our airspace is a clear violation of our sovereignty as well as international law, and it is unacceptable that this has occurred," the official said.

The Pentagon played down the potential value of the balloon for acquiring intelligence, mainly because they do not think the balloon was able to gather the information that couldn't be acquired in other ways, namely by spy satellites.

However, the initial public reaction by Biden administration officials and notable politicians on both sides underscored how delicate relations with Beijing have become.

Republican Senator Tom Cotton had called for Blinken to cancel his trip, while Republican former President Donald Trump, a declared presidential candidate for 2024, posted "SHOOT DOWN THE BALLOON!" on his Truth Social media platform.