UK fighter jets are on standby to shoot down Chinese spy balloons if any are spotted in British air space, the prime minister has said. Rishi Sunak said Typhoon planes were ready at all times in case the UK came under threat from balloons such as the one US officials said they shot down last week. Meanwhile the UK is conducting a security review after four unidentified objects were spotted in US or Canadian air space in just over a week. US fighter jets shot down the fourth of these over Lake Huron on Sunday. Sunak said: “I want people to know that we will do everything it takes to keep the country safe. We have something called the ‘quick reaction alert force’ which involves Typhoon planes which are kept on 24/7 readiness to police our air space.” He added: “I can’t go into detail on national security matters, but we’re in constant touch with our allies and we’ll do whatever it takes to keep our country safe,” he added. Separately, the defence secretary, Ben Wallace, said: “The UK and her allies will review what these airspace intrusions mean for our security. This development is another sign of how the global threat picture is changing for the worse.” On Sunday, the US military shot down the fourth flying object, with the air force general overseeing North American airspace indicating he would not yet rule out any explanation for the objects – including extraterrestrial ones. No 10 would not comment on the origin of the balloons over North America. But asked whether Sunak believed in extraterrestrial life, his spokesperson said: “He is more focused on terrestrial issues.” The high-altitude unidentified object shot down on Sunday was described as an “octagonal structure” with strings attached to it. It is understood to have been the same item that was picked up by radar over Montana on Saturday. It was struck by an air-to-air missile launched by F-16 fighter jets and had been flying across the Great Lakes region at 20,000ft, a height that could have posed a risk to civilian aircraft. The Pentagon said the object appeared to have travelled near US military sites and posed a threat to civilian aviation, as well as being a potential tool for surveillance. Beijing insisted the first balloon shot down by the US on 4 February after crossing its airspace was being used solely for meteorological purposes. But Washington has dismissed this, suspecting it was a spy craft. And the Telegraph reported that the UK’s defence review will show whether or not changes to surveillance of British airspace are necessary to protect against similar craft. The UK transport minister Richard Holden said Chinese spy balloons may have already been deployed over the UK. Asked on Sky News whether it was possible that “Chinese spy balloons have already been used over the UK”, Holden said: “It is possible. It is also possible, and I would think likely, that there would be people from the Chinese government trying to act as a hostile state.” He added: “I think we have to be realistic about the threat these countries pose to the UK.” Alicia Kearns, the chair of the Commons foreign affairs select committee, said: “British airspace is protected by an exceptional quick reaction force, which has proved itself very capable against unknown and hostile aircraft. “The US will share with us and Five Eyes allies its conclusions on the capabilities of the Chinese balloons, and at that point an internal review of our ability to identify, track, disrupt and destroy these balloons will be undertaken.” Wallace has previously said he would have reacted to the first balloon in the same way as the Pentagon did, had it appeared in British skies. That one was followed almost a week later – on Friday – by an unknown “car-sized” object flying in US airspace off the coast of Alaska that the US military again shot down. The Canadian prime minister, Justin Trudeau, said on Saturday that he ordered a US warplane to shoot down an unidentified object that was flying high over northern Canada. Beijing admitted that another balloon spotted flying over Latin America earlier this month was from China.