US secretary of state Antony Blinken has cancelled his weekend visit to China after the Pentagon said it discovered a Chinese spy balloon that has been flying over sensitive nuclear missile sites in the western state of Montana.

The top US diplomat had been expected to meet China’s president Xi Jinping. He would have been the first Biden administration cabinet secretary to visit China and the first secretary of state to travel to the country in more than five years.

Speaking at the state department after cancelling his trip, Blinken said the presence of the Chinese balloon was an “irresponsible act” and a “clear violation of US sovereignty and international law”.

“The People’s Republic of China’s decision to take this action on the eve of my planned visit is detrimental to the substantive discussions that we were prepared to have,” Blinken added.

The about-face came after the Pentagon on Thursday said a Chinese spy balloon had entered US airspace this week and was flying over Montana, where one of the bases that house nuclear intercontinental ballistic missiles is located.

The Pentagon on Friday evening said the US had detected a second Chinese spy balloon over Latin America.

China’s Ministry of Foreign Affairs on Saturday said it was “a matter for the US” to decide when its delegation would visit Beijing, adding that politicians and media had “hyped” up the incident to “attack and smear China”.

Beijing rejected suggestions that the balloon was spying, saying it was a “civilian airship used for research, mainly meteorological, purposes” that deviated from its planned course because of winds and “limited self-steering capability”.

“The Chinese side regrets the unintended entry of the airship into US airspace due to force majeure,” it said.

Pentagon spokesman Brigadier General Patrick Ryder on Friday said the balloon was flying eastward and was over the centre of the US. He declined to say whether China’s military was in control of the balloon, but said US military commanders had determined it posed no physical threat to civilians on the ground.

Asked about China’s explanation, he said: “We know that it’s a surveillance balloon.” He said it would probably stay over the US for a few days.

A senior state department official said Blinken spoke to Wang Yi, China’s top foreign policy official, on Friday to inform him that he was cancelling his trip. The official said the administration had raised concerns about the balloon with China on Wednesday in Washington.

“We’ve been crystal clear with our Chinese counterparts that this was an unacceptable and irresponsible incident,” said the official.

The discovery of the balloon has complicated an attempt by Washington and Beijing to stabilise their turbulent relationship. When US President Biden met China’s Xi on the sidelines of the G20 summit in Bali in November, the leaders agreed that the two powers should attempt to set a floor under the relationship, which has sunk to its lowest level since the countries established diplomatic relations in 1979.

US officials said China had previously flown spy balloons over the country but that this one spent more time overhead. The US said it had taken steps to ensure the balloon could not obtain sensitive military information.

Canada said it was monitoring a potential second ballon incident. Its foreign ministry said it had summoned China’s ambassador to Ottawa to protest against and that it would “continue to vigorously express our position to Chinese officials through multiple channels”.

Mike Gallagher, the Republican head of the new House China committee, and Raja Krishnamoorthi, the panel’s top Democrat, said the Chinese Communist party “should not have on-demand access to American airspace”.

“Not only is this a violation of American sovereignty . . . but it also makes clear that the CCP’s recent diplomatic overtures do not represent a substantive change in policy,” the lawmakers said in a joint statement.

Michael McCaul, the top Republican on the House foreign affairs committee, told the Financial Times that the US should remove the balloon from its airspace. He said US officials had acknowledged they monitored the balloon since it flew over the Aleutian Islands where “it could have easily been shot down over water”.

“Allowing it to remain over US soil not only threatens the privacy of every American, but it sends a powerfully dangerous message to the Chinese Communist party and our other adversaries that this type of aggressive incursion is somehow acceptable,” he said.

President Joe Biden had asked the Pentagon to provide military options regarding the balloon but the administration decided not to shoot it down because of the risk to people on the ground, as well as its assessment that the balloon did not provide China with intelligence that it could not glean by other means, including from low-Earth orbit satellites.