Taiwan's President Tsai Ing-wen waves while registering as the ruling Democratic Progressive Party (DPP) 2020 presidential candidate at the party's headquarter in Taipei on March 21, 2019.

Taiwan's president is expected to transit in the U.S. on Friday for the second time this month, when she returns from visiting diplomatic allies in the Caribbean — a move that will make China very angry.

Tsai Ing-wen, the island's pro-independence leader, is due to make her second stopover in Denver on Friday.

"China opposes official exchange between the US and Taiwan. This position is firm and clear," the Chinese foreign ministry said on July 12. The U.S. should not to allow Tsai's transit and must "stop the official exchange with Taiwan," said Geng Shuang, spokesperson for the Chinese foreign ministry.

The visit comes on the heels of the U.S. State Department recently approving a $2.2 billion sale of weapons to Taiwan— a self-ruled island viewed by Beijing as a breakaway province that has no right to state-to-state ties.

The timing of both is significant and reflects a "much higher risk tolerance from the Donald Trump administration when it comes to growing U.S.-Taiwan ties," said Kelsey Broderick, China analyst at the Eurasia Group, a risk consultancy.

The U.S. State Department sought to downplay Tsai's visit, describing it as "private and unofficial."

Earlier in July, Tsai transited through New York — another major U.S. city — when she was on her way to the Caribbean. That visit saw her meeting members of the U.S. Congress as well as representatives from Taiwan's 17 remaining diplomatic allies, and speaking at Columbia University. She also addressed a 1,000-strong crowd of supporters, according to the Taipei Times.

Her visits come at a low point in U.S.-China relations. In addition to sparring over trade, the world's two biggest economies are also at loggerheads over Taiwan.

Washington's ties with the self-ruled island are technically unofficial. But under the Taiwan Relations Act, "the United States shall provide Taiwan with arms of a defensive character."