Taiwan's president, Tsai Ing-wen, has left for the United States Saturday on her way to Central America, a trip that will be scrutinised by China, incensed by her congratulatory call to Donald Trump.

While the focus of the nine-day trip is to bolster relations with Taiwan's Central American allies, Tsai's US stopovers will be closely watched amid speculation she may make contact with the president-elect and his team.

She is to transit in Houston this weekend and return to Taipei via San Francisco next weekend.

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

"A transit is a transit," the Taiwanese leader told reporters last week, in response to 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.

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Last month, Taiwan's deputy foreign minister, Javier Hou, told a legislative committee that the ministry wanted to arrange meetings with members of the US Congress from both parties as per past protocols, according to Taiwan's state-owned Central News Agency.

The call in December almost upended decades of diplomatic practise in the US. 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 trip to Central America also serves as a way to create strong international relations that could help strengthen Taiwan's self-rule.

Taiwan is considered a breakaway state by China, and it is down to 21 allies, after the small African nation of Sao Tome and Principe switched recognition to Beijing last month.

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".