TAIPEI - A U.S. congressional delegation visiting Taiwan said Friday the U.S. would act if the island was attacked and promised to resolve the $19 billion backlog in its defense purchases from the U.S.

'Know that any hostile unprovoked attack on Taiwan will result in a resolute reaction from the U.S.,' said Rob Wittman, vice chairman of the House Armed Services Committee, in a speech, ahead of meetings with President Tsai Ing-Wen.

U.S. law requires Washington provide Taiwan with the means to defend itself and treat all threats to the island as matters of 'grave concern,' but remains ambiguous on whether it would commit forces in response to an attack from China.

Wittman of Virginia, along with Carlos Gimenez of Florida and Jen Kiggans of Virginia, arrived Thursday for a three-day visit to Taiwan. The three Republicans are meeting with Tsai and the head of Taiwan's National Security Council Wellington Koo.

Taiwan is a self-ruled island claimed by China that has faced increasing military harassment in recent years as Chinese fighter jets and navy ships hold daily exercises aimed at the island, often coming near the island or encircling it. Over the years, to beef up its defense, Taiwan has bought $19 billion in military items from the U.S., but most of that remains undelivered.

'We have an obligation to make sure that we fill the backlog of foreign military sales that exist now between our countries,' Wittman said, adding that both Republicans and Democrats were working on the issue.

The U.S. has started finding new ways to support Taiwan in defense aid. In July, the United States has announced $345 million in military aid in a major package drawing on America's own stockpiles.

On Wednesday, the Biden administration approved the first-ever U.S. military transfer to Taiwan under a program generally reserved for assistance to sovereign, independent states. The amount was modest at $80 million, and officials did not specify what exactly the money would be used for.