BEIJING (AP) — China is protesting the alleged incursion of a U.S. Air Force U-2 spy plane into a no-fly zone imposed during live-fire military exercises in the country’s north.

In a statement issued late Tuesday, the Ministry of National Defense said the action had “seriously interfered in normal exercise activities” and “severely incurred the risk of misjudgment and even of bringing about an unintended air-sea incident.”

“This was a naked act of provocation,” the ministry said, quoting spokesperson Wu Qian. China has lodged a stern protest and demanded the U.S. cease such actions, Wu said.

The statement said the exercises were being staged by China’s Northern Theater Command but gave no details as to their exact time and place. However, the Maritime Safety Administration had announced drills that started Monday and run through Sept. 30 over the Bohai Gulf east of Beijing, an area under the northern command.

Relations between the U.S. and China have sunk to their lowest in decades amid disputes over a myriad of issues including trade, technology, Taiwan and the South China Sea.

The high-altitude reconnaissance planes were flown over China, the former Soviet Union and other countries in the Communist bloc during the Cold War and upgraded versions to continue to support missions in Asia and elsewhere.

China is also staging naval drills in the South China Sea, which it claims virtually in its entirety but over which five other governments also exercise claims. China objects to all U.S. military activity in and over the strategic waterway, especially “freedom of navigation operations" during which U.S. Navy ships sail near to Chinese-held islands.

The defense ministry also announced earlier this month that the Eastern Theater Command held integrated “combat exercises" in the Taiwan Strait and surrounding waters.

The ministry characterized those drills as a “necessary move responding to the current security situation in the Taiwan Strait and were meant to safeguard national sovereignty."

China claims Taiwan, a self-governing democracy and close U.S. ally, as a part of its territory and threatens to use military force to bring it under its control. Washington and Taipei have increased military and governmental contacts in recent years and this month, Health and Human Services Secretary Alex Azar became the highest-ranking U.S. official to visit Taiwan, prompting a Chinese protest.

The Eastern Theater Command will “stay on high alert and take all necessary measures to fight against provocations and protect national sovereignty and territorial integrity," the ministry quoted command spokesperson, Senior Colonel Zhang Chunhui, as saying.