TAIPEI, Taiwan (AP) — Taiwan scrambled fighter jets, put its navy on alert and activated missile systems in response to nearby operations by 34 Chinese military aircraft and nine warships that are part Beijing’s strategy to unsettle and intimidate the self-governing island democracy.

The large-scale Chinese deployment came as Beijing increased preparations for a potential blockade or military action against Taiwan that have stirred increasing concern among military leaders, diplomats and elected officials in the United States (US), Taiwan’s key ally.

In a memo last month, US Air Force General Mike Minihan instructed officers to be prepared for a US -China conflict over Taiwan in 2025. As head of the Air Mobility Command, Minihan has a keen understanding of the Chinese military and his personal remarks echo calls in the US for heightened preparations.

Taiwan’s Defense Ministry said 20 Chinese aircraft on Tuesday crossed the central line in the Taiwan Strait that has long been an unofficial buffer zone between the sides, which separated during a civil war in 1949.

China claims the island republic as its own territory, to be taken by force if necessary, while the vast majority of Taiwanese are opposed to coming under the control of China’s authoritarian Communist Party.

Taiwan’s armed forces “monitored the situation … to respond to these activities,” the Defense Ministry said Wednesday.

That announcement came as NATO Secretary-General Jens Stoltenberg warned that China’s growing assertiveness and collaboration with Russia pose a threat not only to Asia but also to Europe.

On a visit to Japan on Wednesday, Stoltenberg said China is increasingly investing in nuclear weapons and long-range missiles without providing transparency or engaging in arms control talks. Stoltenberg earlier criticised China for “bullying its neighbours and threatening Taiwan” and stressed the need for Japan and other democracies to work together with the alliance to defend the international order.

“NATO needs to make sure we have friends,” he said, citing escalating Chinese attempts to coerce neighbours and threaten Taiwan. “It is important to work more closely with our partners in the Indo Pacific.”

China’s Foreign Ministry responded by accusing NATO of exceeding its mandate and having “played up China’s threats.”

“China is always a force for regional and global peace and stability,” ministry spokesperson Mao Ning said at a regular briefing.

“I would like to stress that the Asia-Pacific is not a battlefield for the geopolitical contest and does not welcome the Cold War mentality and bloc confrontation,” Mao said.

It wasn’t clear what prompted the Chinese action in Taiwan, although it came just ahead of a visit to Beijing by US Secretary of State Antony Blinken, who would become the highest-ranking official to visit China since President Joe Biden’s election in 2020.

Beijing frequently seeks to flag Taiwan as the most serious issue in US-China relations ahead of top-level discussions, leading then to discussions of other economic, trade and political issues where there is more room for meaningful exchanges.

China has sent warships, bombers, fighter jets and support aircraft into airspace near Taiwan on a near-daily basis, hoping to wear down the island’s limited defence resources and undercut support for pro-independence President Tsai Ing-wen.

Chinese fighter jets have also confronted military aircraft from the US and allied nations over international airspace in the South China and East China seas, in what Beijing has described as dangerous and threatening manoeuvres.

A string of visits in recent months by foreign politicians to Taiwan, including by then-US House Speaker Nancy Pelosi and numerous politicians from the European Union, spurred displays of military might from both sides.

In response to Pelosi’s visit in August, China staged war games surrounding the island and fired missiles over it into the Pacific Ocean.

China has repeatedly threatened retaliation against countries seeking closer ties with Taiwan, but its attempts at intimidation have sparked a backlash in popular sentiment in Europe, Japan, the US and other nations.

READ: China reaffirms threat of military force to annex Taiwan

Taiwan is set to hold presidential elections next year, in contrast to China’s system of total control by President and party General Secretary Xi Jinping, who has removed term limits to effectively make him leader for life. China’s efforts to reach out to Taiwan’s pro-unification Nationalist Party have largely backfired.