Tensions between China and Taiwan are at their worst in over four decades according to Taiwan’s defense minister, citing a recent increase in incursions by Bei Jing’s airforce into the island’s air defense identification zone reports NPR.

Although no actual shots have been fired, Taipei said that close to 150 aircraft that belong to China’s People’s Liberation Army Air Force entered the zone during a period of four days starting Friday, as part of what Taiwan says is a strategy of harassment.

Taiwan’s Minister of Defense Chiu Kuo-cheng said that the situation is “the most serious” in the over 40 years since he enlisted in the military.

Taiwan is an autonomous island boasting roughly 24 million people. It is situated near China’s coast. Beijing claims that it is part of its territory.

Washington said this week that it is communicating with Taiwan regarding the incursions and is “conveying clear messages through diplomatic channels,” according to White House press secretary Jen Psaki.

“We remain concerned by the People’s Republic of China’s provocative military activity near Taiwan, which is destabilizing, risks miscalculations and undermines regional peace and stability,” she said at a press conference on Monday, stressing that American commitment to Taipei is “rock solid.”

On Tuesday, US President Joe Biden said that he spoke with Chinese President Xi Jinping on Taiwan and that they have agreed to adhere to the “Taiwan agreement,” an apparent reference to the long-standing policy whereby the US recognizes Beijing’s authority over Taipei so long as China does not attack the island.

Australia and Japan have also reportedly called on China and Taiwan to hold discussions.

On Tuesday a, Taiwan reported a record 56 Chinese aircraft flew into its air identification zone in just one 24-hour period.

“Taiwan must be on alert. China is more and more over the top,” Premier Su Tseng-chang told reporters in Taipei on Tuesday, adding that the island must “strengthen itself” against the external threat.