Republican Senator Roger Wicker, a vocal supporter of Taiwan, is leading a congressional delegation trip in the Asia-Pacific region, which has included stops in Hawaii, Guam, Palau and the Philippines.

Their arrival in Taipei comes 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.

"We stand here to re-emphasise the partnership and the security, friendship agreement that the United States has had with Taiwan for some decades," Wicker told reporters ahead of meetings with President Lai Ching-te and other government officials.

"We have re-emphasised that in the past two National Defense Authorization acts, and we will add to the provisions again in this year's Defense Authorization Act, which will be on the floor of the Senate on Tuesday of next week."

Republican Senator Deb Fischer, who also sits on the armed services committee, said "security, opportunities and progress for this part of the world" would be on the agenda during their two-day visit to Taiwan.

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

While the United States does not recognise Taiwan as a country, Washington is its most important backer and biggest supplier of arms.

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.