US Secretary of State Antony Blinken has postponed his planned trip to China following the discovery of a high-altitude Chinese balloon flying over the western United States, a senior State Department official said on Friday, adding that Blinken would seek to travel to China "when conditions allow."

China earlier expressed regret that what it called a "civilian" airship had strayed into US territory after being blown off course, an incident that sparked a political furore in the United States.

"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 State Department official told reporters on a call.

"After consultations with our interagency partners as well as with Congress, we have concluded that the conditions are not right at this moment for Secretary Blinken to travel to China," the official said.

In a statement released by the Foreign Ministry late on Friday, officials described the object as a "civilian airship used for research, mainly meteorological, purposes".

"Affected by the westerlies and with limited self-steering capability, the airship deviated far from its planned course," the ministry added.

"The Chinese side regrets the unintended entry of the airship into US airspace due to force majeure." (Reuters)