TAIPEI (AFP) -- Taiwan's President Tsai Ing-wen left for the United States Saturday on her way to Central America, a trip that will be closely watched by Beijing, incensed by her congratulatory call to Donald Trump.

While the focus of the nine-day trip is to visit Taiwan's ally nations in Central America, Beijing-sceptic Tsai's transit stops in Houston this weekend and San Francisco next weekend will be under scrutiny as cross-strait tensions rise.

Tsai's conversation with Trump in December after he won the presidency upended decades of diplomatic precedent in which Washington has effectively ignored Taipei in favour of Beijing, which considers Taiwan a breakaway province to be brought back within its fold.

Since then, China has stepped up military drills near Taiwan, with speculation its sole aircraft carrier may pass through the Taiwan Strait during or shortly after Tsai's trip.

The drills are seen as a show of strength by Beijing as its ties with the self-ruled island and the US deteriorate.

Beijing has asked Washington to bar Tsai from flying through US airspace.

"A transit is a transit," the Taiwanese leader told reporters last week, when asked whether she would be meeting anyone from Trump's administration.

Trump himself appeared to have ruled out meeting Tsai this trip, saying it is "a little bit inappropriate" to meet anybody until he takes office January 20.

Taiwan's presidential office declined to provide details of Tsai's itinerary during her US stays.

"What China cares most about is whether Tsai and Trump will meet," political analyst Liao Da-chi told AFP.

"These are all warning signals to see how Taiwan will respond, as well as testing waters with the US," added Liao, a professor at the National Sun Yat-sen University.

Tsai will visit Honduras, Nicaragua, Guatemala, and El Salvador -- among the dwindling number of states that officially recognise Taiwan.

She will attend the presidential inauguration in Nicaragua on Tuesday and meet with the heads of states of the other three nations.

Taiwan is down to 21 allies after the small African nation of Sao Tome and Principe switched recognition to Beijing last month.

The Vatican is Taiwan's highest-profile supporter, but the Holy See's recently improving relations with Beijing is casting doubt over ties with the island.

Analyst Liao said Beijing would continue to woo the island's allies as a pressure tactic on Tsai, who refuses to acknowledge the concept that there is only "one China".

Taiwan's defence ministry has declined to comment on speculation that China's Liaoning aircraft carrier may pass through the Taiwan Strait on its return journey after its first exercise in the Pacific.

The ministry is "monitoring at all times" the position of the Liaoning, spokesman Chen Chung-chi said.

The vessel and five other warships passed south of the island outside Taiwan's air defence identification zone last month, before heading to the South China Sea.