The US shot down a balloon off the Carolina coast after it traversed sensitive military sites across North America. China insisted the flyover was an accident involving a civilian aircraft.

Vice Foreign Minister Xie Feng said he lodged a formal complaint with the US Embassy on Sunday over the “US attack on a Chinese civilian unmanned airship by military force.”

“However, the United States turned a deaf ear and insisted on indiscriminate use of force against the civilian airship that was about to leave the United States airspace, which obviously overreacted and seriously violated the spirit of international law and international practice,” Xie said.

The presence of the balloon in the skies above the US dealt a severe blow to already strained US-Chinese relations that have been in a downward spiral for years. It prompted Secretary of State Antony Blinken to abruptly cancel a high-stakes Beijing trip aimed at easing tensions.

Xie repeated China’s insistence that the balloon was a Chinese civil unmanned airship that blew into US mistake, calling it “an accidental incident caused by force majeure.”

China would “resolutely safeguard the legitimate rights and interests of Chinese companies, resolutely safeguard China’s interests and dignity and reserve the right to make further necessary responses,” he said.

US President Joe Biden issued the shootdown order after he was advised that the best times for the operation would be when it was over water, US officials said. Military officials determined that bringing down the balloon over land from an altitude of 60,000 feet (18,000 meters) would pose an undue risk to people on the ground.

“What the US has done has seriously impacted and damaged both sides’ efforts and progress in stabilizing Sino-US relations since the Bali meeting,” Xie said, referring to the recent meeting between Biden and his Chinese counterpart Xi Jinping in Indonesia that many hoped would create positive momentum for improving ties that have spiraled to their lowest level in years.

The sides are at odds over a range of issues from trade to human rights, but Beijing is most sensitive over alleged violations by the US and others of its sovereignty and territorial integrity.

Beijing strongly protests military sales to Taiwan and visits by foreign politicians to the island, which it claims as Chinese territory to be recovered by force if necessary.

It reacted to a 2022 visit by then-US House Speaker Nancy Pelosi by firing missiles over the island and staging threatening military drills seen as a rehearsal for an invasion or blockade. Beijing also cut off discussion with the US on issues including climate change that are unrelated to military tensions.

Last week, a Foreign Ministry spokesperson warned Pelosi’s successor, House Speaker Kevin McCarthy, not to travel Taiwan, implying China’s response would be equally vociferous.

“China will firmly defend its sovereignty, security and development interests,” Mao Ning said. McCarthy said China had no right to dictate where and when he could travel.

China also objects when foreign military surveillance planes fly off its coast in international airspace and when US and other foreign warships pass through the Taiwan Strait, accusing them of being actively provocative.

In 2001, a US Navy plane conducting routine surveillance near the Chinese coast collided with a Chinese fighter plane, killing the Chinese fighter pilot and damaging the American plane, which was forced to make an emergency landing at a China naval air base on the southern Chinese island province of Hainan.

China detained the 24-member US Navy aircrew for 10 days until the US expressed regret over the Chinese pilot’s death and for landing at the base without permission.

The South China Sea is another major source of tension. China claims the strategically key sea virtually in its entirety and protests when US Navy ships sail past Chinese military features there.

At a news conference Friday with his South Korean counterpart, Blinken said “the presence of this surveillance balloon over the United States in our skies is a clear violation of our sovereignty, a clear violation of international law, and clearly unacceptable. And we’ve made that clear to China.”

“Any country that has its airspace violated in this way I think would respond similarly, and I can only imagine what the reaction would be in China if they were on the other end,” Blinken said.

China’s weather balloon excuse should be dismissed outright, said Oriana Skylar Mastro, an expert on Chinese military affairs and foreign policy at Stanford University.

“This is like a standard thing that countries often say about surveillance assets,” Mastro said.

China may have made a mistake and lost control of the balloon, but is was unlikely to have been a deliberate attempt to disrupt Blinken’s visit, Mastro said.

For the US administration, the decision to go public and then shoot down the balloon marks a break from its usual approach of dealing with Beijing on such matters privately, possibly in hopes of changing China’s future behavior.

However, Mastro said, it was unlikely that Beijing would respond positively.

“They’re probably going to dismiss that and continue on as things have been. So I don’t see a really clear pathway to improved relations in the foreseeable future.”