Taiwan is a "trusted and capable" partner of the United States and Taipei's global relationships, including with Eswatini, provide significant benefits, the US State Department said of President Lai Ching-te's trip to the southern African kingdom.

Lai arrived in the former Swaziland on Saturday on a surprise visit after his government blamed Chinese pressure for nixing an earlier trip planned for last month by getting three Indian Ocean states to deny overflight permission for his aircraft.

China views democratically governed Taiwan as part of its territory with no right to state-to-state ties, a position Taiwan's government strongly disputes, and Beijing has demanded countries stop any engagements with the island.

The United States is Taiwan's most important international backer and arms supplier, to the constant anger of Beijing.

"Taiwan is a trusted and capable partner of the United States and many others, and its relationships around the world provide significant benefits to the citizens of those countries, including Eswatini," a State Department spokesperson said.

Every democratically elected Taiwan president has made overseas trips to visit Taiwan's diplomatic partners, and Lai's predecessor Tsai Ing-wen went to Eswatini in 2023 and 2018, the spokesperson added. 

"This travel is routine and should not be politicised."

Eswatini, home to around 1.3-million people, is one of just 12 countries with formal diplomatic ties to Taiwan, and the only one in Africa, where China has deep economic relationships.

"When we go with the spirit of the UN, which says the UN is not going to leave no one behind, there are those who are still left behind - 23-million people of Taiwan still feel that they are left behind," King Mswati III told a banquet for Lai on Sunday night, in video footage Lai's office released on Monday.

Taiwan is not a United Nations member due to China's objections.

China has condemned Lai's trip, which he made on an Eswatini government aircraft, saying he was "like a rat scurrying across the street".

Taiwan is "the biggest point of risk" for Sino-US relations, Chinese Foreign Minister Wang Yi told US Secretary of State Marco Rubio on Thursday, ahead of a summit between the Chinese and US leaders in Beijing later this month.

Lai says that only the island's people can decide their future and that Taiwan has a right to engage with the rest of the world.

Lai had planned his trip to Eswatini for the 40th anniversary of the king's accession in April.