A small group of special US forces and a unknown number of marines have been working with Taiwanese troops for at least a year, according to a report by The Wall Street Journal.
US special operations forces have been quietly training Taiwanese troops for months, risking the ire of China, a Pentagon official has said.
A contingent of around 20 special operations and conventional forces has been conducting the training for less than a year, the official, who declined to be identified, told AFP, adding that some of the trainers rotate in and out.
The official largely confirmed a Wall Street Journal report which said the training has been going on for at least a year, amid China's rising verbal threats against the island ally of the United States.
Taiwan's Defense Ministry declined to comment on the report, but Pentagon spokesman John Supple said that generally speaking, US support for Taiwan's military is gauged on its defence needs.
"Our support for and defense relationship with Taiwan remains aligned against the current threat posed by the People’s Republic of China," Supple said in a statement.
"We urge Beijing to honor its commitment to the peaceful resolution of cross-Strait differences."
Taiwan’s Premier Su Tseng-chang said "a just cause always attracts much support" when asked to comment on the Wall Street Journal report on Friday.
"We are making every effort to defend our national sovereignty and our people as well as maintaining regional peace. We are doing all we can and we appreciate like-minded countries working together," he said.
Taiwan media reported last November, citing Taiwan's Naval Command, that US troops had arrived there to train Taiwan marines and special forces in small-boat and amphibious operations.
But those reports were subsequently denied by US and Taiwanese officials, who emphasised the two sides are only involved in bilateral military exchanges and cooperation.
The United States supplies weapons to Taiwan, including missiles for defence and fighter jets, amid Beijing's threat to forcibly retake control of the island and reintegrate it with China.
The US also maintains an ambiguous commitment to defend Taiwan, which Beijing considers a renegade province.
A video released last year and featured in Taiwan media showed US troops taking part in an exercise on the island dubbed "Balance Tamper."
Military tension at highest in four decades
Chinese forces have stepped up their activities toward Taiwan in the past year, conducting sea assault exercises and flying large sorties of bombers and fighters close to Taiwan airspace.
Taiwan's defence minister said Wednesday that military tensions between the island and China are at their highest in four decades, after around 150 Chinese warplanes – a record number – made incursions into its air defence zone in recent days.
He warned that even "slight carelessness" or "miscalculation" could spark a crisis, and that Beijing would be in a position to launch a full-scale invasion in four years.
US State Department Spokesman Ned Price called the Chinese activity "destabilizing" and "provocative."
"We strongly urge Beijing to cease its military, diplomatic and economic pressure and coercion against Taiwan," he said, calling US commitment to the island "rock-solid".
"Let me just say this, we are going to take action now to try to prevent that day from ever coming to pass," national security adviser Jake Sullivan told the BBC when asked if Washington was prepared to take military actions to defend Taiwan.
'Taiwan does not seek military confrontation'
Taiwan does not seek military confrontation, but will do whatever it takes to defend its freedom, President Tsai Ing-wen said on Friday.
Taiwan has complained for more than a year of such activities, which it views as "grey zone warfare", designed to wear out Taiwan's armed forces and test their ability to respond.
"Taiwan does not seek military confrontation," Tsai told a security forum in Taipei.
"It hopes for a peaceful, stable, predictable and mutually-beneficial coexistence with its neighbours. But Taiwan will also do whatever it takes to defend its freedom and democratic way of life."