Taiwan President Tsai Ing-wen was due to leave for the United States on Wednesday, a stop on her way to firm ties with Guatemala and Belize after China snapped up another of the self-ruled island's few diplomatic allies last week.

Belize and Guatemala are among just 13 countries that officially recognise democratic Taipei over Beijing, after Honduras opened diplomatic relations with China on Sunday.

China claims the self-ruled island as part of its territory, to be retaken one day -- by force, if necessary. Under its "One China" principle, no country may maintain official ties with both Beijing and Taipei.

After first visiting New York, President Tsai will continue her 10-day trip by meeting her Guatemalan counterpart Alejandro Giammattei and Belize Prime Minister John Briceno in their respective countries, her office said.

She will then stop in Los Angeles on her way back to Taiwan.

US House Speaker Kevin McCarthy has said he will meet Tsai in California, a move protested by Beijing. Taiwanese authorities have not confirmed the meeting with McCarthy, or Tsai's itinerary while in New York.

Last year, a visit by McCarthy's predecessor to Taiwan sparked an angry response from Beijing, with the Chinese military conducting drills at an unprecedented scale around the island. Taipei said the exercises were preparation for an invasion.

On Wednesday, China's Taiwan Affairs Office called Tsai's US stop "a provocation".

"This so-called transit by the Taiwan leader is essentially a provocation that aims to seek independence relying on the US," said Zhu Fenglian, a spokesperson.

A senior US official later said that China should not use Tsai's stopover as a "pretext" for aggression against the island.

"There's absolutely no reason for China to use that as a pretext to overreact or to engage in further coercion directed at Taiwan," the senior administration official told reporters on condition of anonymity, adding that the stopover did not represent a change in US policy.

- Official vs unofficial ties -

Analysts say the Taiwanese president's US stopover comes at a key time, with Beijing having ramped up military, economic and diplomatic pressure on the island since Tsai came to power in 2016, poaching nine of its diplomatic allies since then.

"Beijing's attempts to poach Taiwan's diplomatic partners will lead to Taiwan developing closer ties with the United States," said James Lee, a researcher on US-Taiwan relations at Taiwan's Academia Sinica.

The United States remains Taiwan's most important international ally -- and its biggest arms supplier -- despite having switched its own diplomatic recognition to Beijing in 1979.

Lee said the strength of Taiwan's unofficial allegiances was as important as any official ties.

"The loss of official relations with third countries will be offset by a deepening of Taiwan's unofficial relations," he said.

China bristles not only at official exchanges, but also at visits by politicians from countries with whom Taiwan has unofficial relations.

Recent visits by a Czech delegation and a German minister were met with rebukes from Beijing.

- Diplomatic battleground -

Tsai heads to Latin America at a time when China has upped investment in the region, which has been a key diplomatic battleground between Taipei and Beijing since the two sides split in 1949 after a civil war.

On Sunday, Taiwan accused China of using "coercion and intimidation" to lure away its allies, after Honduran Foreign Minister Enrique Reina and his Chinese counterpart Qin Gang officially launched relations in Beijing.

Honduras, one of the poorest countries in the region, made the switch due to economic necessity, Reina had said earlier.

The move continued a trend in Latin America, with Nicaragua, El Salvador, Panama, the Dominican Republic and Costa Rica all switching diplomatic recognition to Beijing in recent years.

In addition to Guatemala and Belize, Taiwan still has official ties with a handful of countries in Latin America and the Caribbean, including Paraguay and Haiti. Its other allies are mostly small islands in the Pacific, and the Vatican.