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 defense 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 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 emphasized the two sides are only involved in bilateral military exchanges and cooperation.

The United States supplies weapons to Taiwan, including missiles for defense 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."

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.

On Monday, Taiwan scrambled its own air force after a record 56 Chinese warplanes crossed into its air defense zone.

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".