Republican senators Roger Wicker and Deb Fischer arrived in Taipei on Friday for a two-day visit, as US President Donald Trump seeks to strike a trade deal with China -- which insists Taiwan is part of its territory and has threatened to use force to annex it.

Since Trump returned to the White House in January, there have been growing jitters in Taipei over the strength of the Taiwan-US relationship and Washington's willingness to defend the island if China were to attack.

Wicker, who chairs the powerful armed services committee and is a vocal supporter of Taiwan, said he and Fischer wanted to reiterate to Taiwan "our determination to remain the best of friends and to defend the freedom of everyone and both of our great countries".

"It is our determination and our intention that Taiwan remain free and make its own decisions," Wicker said after talks with Taiwanese President Lai Ching-te.

"Part of maintaining the freedoms that we have is enhanced cooperation militarily, enhanced cooperation with our defence industrial base, making the best use of those funds."

Fischer said the Senate understands "the gravity of the challenges that Taiwan faces" and that a "stronger Taiwan means a stronger United States and vice versa".

While the United States stopped recognising Taiwan in the late 1970s in favour of China, Washington has remained Taipei's most important backer and biggest supplier of arms that it would need to defend itself.

Ahead of the meeting with Wicker and Fischer, Lai said he hoped Taiwan and the United States will further "enhance cooperation", and insisted the island and China were "not subordinate" to each other.

Wicker and Fischer have been travelling in the Asia-Pacific region for the past week, stopping in Hawaii, Guam, Palau and the Philippines.

US-Taiwan ties have been strained since Trump took office and launched a global trade war and pressured governments in Europe and elsewhere to spend more on their own defence.

The Trump administration reportedly denied permission for Lai to transit in New York as part of a planned official trip to Latin America this month after Beijing objected.

Lai reportedly then cancelled the trip.

Taiwan is also struggling to finalise a tariff deal with the United States after Washington imposed a temporary 20 percent levy that has alarmed the export-dependent island's manufacturers.

As those negotiations continue, Lai's government has announced plans to increase defence spending to more than three percent of GDP next year and to five percent by 2030.