China has firmly rejected allegations of espionage relating to a balloon that is drifting in US airspace, as the issue places growing strain on already troubled relations between the two nations. US Secretary of State Antony Blinken called the intrusion of the 'surveillance balloon' into US airspace 'unacceptable' and 'irresponsible.' China on Saturday denied allegations of spying, saying the aircraft involved was a research balloon that had been blown off course by 'force majeure.' 'We don't accept baseless speculation and sensationalism,' Beijing's Foreign Ministry said on Saturday, citing senior diplomat Wang Yi's telephone conversation with Blinken a day earlier. Blinken cancelled his visit to Beijing, planned for Sunday. It would have been the first visit to China by a US secretary of state since 2018. Blinken stressed that the US wanted to keep the channels of communication open with Beijing and that the visit should go ahead without delay 'when conditions permit.' A Chinese Foreign Ministry spokesman reiterated that the balloon over the US had been used 'for meteorological and other scientific research.' 'Due to the west wind drift and because of limited steering capabilities, the airship went far off the planned route.' The Pentagon did not accept the claim however. 'We know it's a surveillance balloon,' Ryder said. The Department of Defence had made public the sighting of the first balloon on Thursday evening. The balloon was spotted over Montana, where 150 intercontinental ballistic missiles armed with nuclear warheads are stored at a military base in the north, according to a report.