“The U.S. special-operations deployment is a sign of concern within the Pentagon over Taiwan’s tactical capabilities in light of Beijing’s yearslong military buildup and recent threatening moves against the island,” the  WSJ reported.

News that U.S. forces have been training Taiwan’s military comes as China has been increasingly belligerent towards the island democracy, which Beijing considers a renegade province of the mainland some 70 years after a civil war forced the then-ruling Kuomintang, or the Chinese Nationalist Party, to flee to what was at one time called the Republic of Formosa. Today, the Kuomintang is the leading opposition party in Taiwan.

Over the past two weeks, China has alarmed Taiwanese and U.S. officials by sending nearly 150 aircraft near the island including fighters, nuclear-capable bombers, and anti-submarine aircraft — a record for those kinds of sorties, according to government officials in Taipei.

The increase in military activity by China led President Joe Biden to discuss the situation with his Chinese counterpart, Xi Jinping, last month, in which he said both men consented to abide by the “Taiwan agreement.”

“I’ve spoken with Xi about Taiwan. We agree we will abide by the Taiwan agreement,” Biden told reporters. “That’s where we are and I made it clear that I don’t think he should be doing anything other than abiding by the agreement.”

But that phone call reportedly took place Sept. 9, well before China’s latest acts of aggression. And Biden’s remark about a Taiwan agreement was also baffling to others.

“It’s not exactly clear what Biden meant by the ‘Taiwan agreement,’ because no formal agreement by that name exists,” the Taiwan News reported. “Analysts were left to speculate that he might mean the Taiwan Relations Act, which was an act passed by Congress in 1979 that authorized de facto diplomatic relations with Taiwan and had a provision that the U.S. should provide weaponry for its defense.”

The agreement also calls on China and Taiwan to solve their differences peacefully, though Xi and other Chinese leaders have often talked about retaking Taiwan militarily to reunify with the mainland.

To that point, Chiu Kuo-cheng, Taiwan’s defense minister, said on Wednesday that China would be capable of launching an attack on the island nation by 2025 and suffer minimal losses.

The Chinese foreign ministry, meanwhile, said that the U.S. should stop providing military assistance to Taiwan and abide by previous agreements. “China will take all necessary steps to protect its sovereignty and territorial integrity,” the statement said.

Media reports last year suggesting that the U.S. had sent a contingent of Marines to Taiwan were never confirmed. The WSJ is the first to report the additional presence of Special Forces troops.

While the United States has sold Taiwan billions of dollars worth of military hardware in recent years, experts believe the country must begin investing more in its own defense and do so more wisely.

“Taiwan badly neglected its national defense for the first 15 years or so of this century, buying too much expensive equipment that will get destroyed in the first hours of a conflict, and too little in the way of cheaper but lethal systems—antiship missiles, smart sea mines and well-trained reserve and auxiliary forces—that could seriously complicate Beijing’s war plans,” Matt Pottinger, a distinguished visiting fellow at Stanford University’s Hoover Institution and a deputy national security adviser during the Trump administration, told the WSJ.

He added that Taiwan’s military spending was similar to that of Singapore, a city-state with a fraction of the island’s population that also “doesn’t have China breathing down its neck.”